Peggle - awesome game and I suggest you get hold of Steam and create an account, just try not to spend too much on it.
http://store.steampowered.com/about/
Fallout 3 - great FPS/RPG, really enjoying this.
Half-life 2 - Great game, love TF2 addon.
If you have to go with some online massive multiplayer type World of warcraft game the ones I suggest trying are:
Eve Online - great game, personally I found it boring though. However it does have free trial.
World of Warcraft - Largest MMORPG, it's good but probably not the best. However it's got a free trial too which at least lets you try it!
Warhammer online - If you don't fancy playing constantly then this is a decent alternative to WoW. No free trial yet though. Personally prefer it to WoW, but many wouldn't agree
Others worth mentioning:
Medievel II:Total War - Incredible game.
Grand Theft Auto 4 - Decent stress relief.
Civilisation 4 - Classic game.