Team Fortress 2

I might have an idea since you said dual-boot....do you have a mac and did dual boot xp on it? Cause you need a med range PC to run this game. Post what you're computer specs if you have a PC, but if you do have a mac, as far as I know, it won't run on it. You need a decent cpu and graphics card to run this game smooth.

Windows likes to create a thumbnail view of video files. For large files this can cause your system to hang.
To fix this problem first download ShellExView (http://www.snapfiles.com/get/shellexview.html) then find the extension named "Video Thumbnail Extractor" and disable it.
And in case this doesn't fix it, I run Windows XP using boot camp on my Mac and can run TF2 perfectly. So if this doesn't work you know this isn't you're problem. HOWEVER Parallels has a problem with using DirectX, so if you are running Windows using parallels then it could be the problem.

Do what klopp does, update your drivers, goto X:\Program Files\Steam\steamapps\username\TF2 and delete the folder. Make sure steam is turned off. Start steam and run TF2 it will "reinstall" the game from the .gfc files and hopefully fix your problem.

But also what mac do you have? One of the new macs on an old one because as minamonster said, the old macs wont play the game too well

One of your ram might be busted, try to pass a memtest on your pc.