(Mostly) unmodified code, courtesy of Phil Harding (see references below for original post).

using System;
using System.ComponentModel;
using System.Runtime.InteropServices;
using System.Security.Principal;

namespace Tools.Network
{
public enum LogonType
{
LOGON32_LOGON_INTERACTIVE = 2,
LOGON32_LOGON_NETWORK = 3,
LOGON32_LOGON_BATCH = 4,
LOGON32_LOGON_SERVICE = 5,
LOGON32_LOGON_UNLOCK = 7,
LOGON32_LOGON_NETWORK_CLEARTEXT = 8,// Win2K or higher
LOGON32_LOGON_NEW_CREDENTIALS = 9// Win2K or higher
};

public enum LogonProvider
{
LOGON32_PROVIDER_DEFAULT = 0,
LOGON32_PROVIDER_WINNT35 = 1,
LOGON32_PROVIDER_WINNT40 = 2,
LOGON32_PROVIDER_WINNT50 = 3
};

public enum ImpersonationLevel
{
SecurityAnonymous = 0,
SecurityIdentification = 1,
SecurityImpersonation = 2,
SecurityDelegation = 3
}

class Win32NativeMethods
{
[DllImport("advapi32.dll", SetLastError = true)]
public static extern int LogonUser(string lpszUserName,
string lpszDomain,
string lpszPassword,
int dwLogonType,
int dwLogonProvider,
ref IntPtr phToken);

[DllImport("advapi32.dll", CharSet = CharSet.Auto, SetLastError = true)]
public static extern int DuplicateToken(IntPtr hToken,
int impersonationLevel,
ref IntPtr hNewToken);

[DllImport("advapi32.dll", CharSet = CharSet.Auto, SetLastError = true)]
public static extern bool RevertToSelf();

[DllImport("kernel32.dll", CharSet = CharSet.Auto)]
public static extern bool CloseHandle(IntPtr handle);
}

/// <summary>
/// Allows code to be executed under the security context of a specified user account.
/// </summary>
/// <remarks>
///
/// Implements IDispose, so can be used via a using-directive or method calls;
///...
///
///var imp = new Impersonator( "myUsername", "myDomainname", "myPassword" );
///imp.UndoImpersonation();
///
///...
///
/// var imp = new Impersonator();
///imp.Impersonate("myUsername", "myDomainname", "myPassword");
///imp.UndoImpersonation();
///
///...
///
///using ( new Impersonator( "myUsername", "myDomainname", "myPassword" ) )
///{
///...
///
///...
///}
///
///...
/// </remarks>
public class Impersonator : IDisposable
{
private WindowsImpersonationContext _wic;

/// <summary>
/// Begins impersonation with the given credentials, Logon type and Logon provider.
/// </summary>
/// <param name="userName">Name of the user.</param>
/// <param name="domainName">Name of the domain.</param>
/// <param name="password">The password. <see cref="System.String"/></param>
/// <param name="logonType">Type of the logon.</param>
/// <param name="logonProvider">The logon provider. <see cref="Mit.Sharepoint.WebParts.EventLogQuery.Network.LogonProvider"/></param>
public Impersonator(string userName, string domainName, string password, LogonType logonType, LogonProvider logonProvider)
{
Impersonate(userName, domainName, password, logonType, logonProvider);
}

/// <summary>
/// Begins impersonation with the given credentials.
/// </summary>
/// <param name="userName">Name of the user.</param>
/// <param name="domainName">Name of the domain.</param>
/// <param name="password">The password. <see cref="System.String"/></param>
public Impersonator(string userName, string domainName, string password)
{
Impersonate(userName, domainName, password, LogonType.LOGON32_LOGON_INTERACTIVE, LogonProvider.LOGON32_PROVIDER_DEFAULT);
}

/// <summary>
/// Initializes a new instance of the <see cref="Impersonator"/> class.
/// </summary>
public Impersonator()
{}

/// <summary>
/// Performs application-defined tasks associated with freeing, releasing, or resetting unmanaged resources.
/// </summary>
public void Dispose()
{
UndoImpersonation();
}

/// <summary>
/// Impersonates the specified user account.
/// </summary>
/// <param name="userName">Name of the user.</param>
/// <param name="domainName">Name of the domain.</param>
/// <param name="password">The password. <see cref="System.String"/></param>
public void Impersonate(string userName, string domainName, string password)
{
Impersonate(userName, domainName, password, LogonType.LOGON32_LOGON_INTERACTIVE, LogonProvider.LOGON32_PROVIDER_DEFAULT);
}

/// <summary>
/// Impersonates the specified user account.
/// </summary>
/// <param name="userName">Name of the user.</param>
/// <param name="domainName">Name of the domain.</param>
/// <param name="password">The password. <see cref="System.String"/></param>
/// <param name="logonType">Type of the logon.</param>
/// <param name="logonProvider">The logon provider. <see cref="Mit.Sharepoint.WebParts.EventLogQuery.Network.LogonProvider"/></param>
public void Impersonate(string userName, string domainName, string password, LogonType logonType, LogonProvider logonProvider)
{
UndoImpersonation();

IntPtr logonToken = IntPtr.Zero;
IntPtr logonTokenDuplicate = IntPtr.Zero;
try
{
// revert to the application pool identity, saving the identity of the current requestor
_wic = WindowsIdentity.Impersonate(IntPtr.Zero);

// do logon & impersonate
if (Win32NativeMethods.LogonUser(userName,
domainName,
password,
(int)logonType,
(int)logonProvider,
ref logonToken) != 0)
{
if (Win32NativeMethods.DuplicateToken(logonToken, (int)ImpersonationLevel.SecurityImpersonation, ref logonTokenDuplicate) != 0)
{
var wi = new WindowsIdentity(logonTokenDuplicate);
wi.Impersonate();// discard the returned identity context (which is the context of the application pool)
}
else
throw new Win32Exception(Marshal.GetLastWin32Error());
}
else
throw new Win32Exception(Marshal.GetLastWin32Error());
}
finally
{
if (logonToken != IntPtr.Zero)
Win32NativeMethods.CloseHandle(logonToken);

if (logonTokenDuplicate != IntPtr.Zero)
Win32NativeMethods.CloseHandle(logonTokenDuplicate);
}
}

/// <summary>
/// Stops impersonation.
/// </summary>
private void UndoImpersonation()
{
// restore saved requestor identity
if (_wic != null)
_wic.Undo();
_wic = null;
}
}
}

References
http://platinumdogs.wordpress.com/2008/10/30/net-c-impersonation-with-network-credentials/