Here is a recent article about a current trojan running through facebook.
Facebook Users Attacked by Trojan Virus :: Schipul Blog

I suggest you start you computer in safe mode and run current versions of anti-spyware and anti-virus software.

Here are a couple I like.
AVG Free - Download AVG Anti-Virus Free Edition for Windows XP and Vista
The home of Spybot-S&D!
Ad-Aware 2008 Free - Lavasoft

To boot in safe mode. A description of the Safe Mode Boot options in Windows XP

1.Restart your computer and start pressing the F8 key on your keyboard. On a computer that is configured for booting to multiple operating systems, you can press the F8 key when the Boot Menu appears.

2.Select an option when the Windows Advanced Options menu appears, and then press ENTER.

3.When the Boot menu appears again, and the words "Safe Mode" appear in blue at the bottom, select the installation that you want to start, and then press ENTER.