The built-in Remote Assistance facilities in Windows XP and Vista work well (if you have XP or Vista).
VNC provides a great alternative:
VNC is available from this website.
If you have Windows XP or an earlier version, choose the "Free" edition, unless you have specific security concerns or plan to leave the software running permanently, in which case the tighter security of the other (paid) versions might be preferred. For occasional use, the free version is normally sufficiently secure (ask for advice). Registration is optional, as is a donation, although this is a great way to support the developers.
If you have Windows Vista, choose the (inexpensive) "Personal" edition. There is also a version for the Mac. Trial licenses are available.
Download the software now. Choose the appropriate edition, and choose the Executable rather than the Zip Archive. Save the file to your desktop.
You'll need to have Administrator privileges to install VNC. If in doubt, you probably do. You'll see an error message if you don't - in which case ask us for advice.
If you want to be on the receiving end of assistance, you need to install the server component, which sends us images of your desktop. Installation is normally quite quick. The following pages take you through this simple process, using the Free edition.