Currently, what is the best game for a computer. I have a windows xp if that helps any.

mafia is good and grand theft auto 1,2,3 and vice city are also good

also if you like fantasy type games, grab neverwinter nights. in my opinion its the best game out there. also, even though its 4 years old, diablo2 is very good.

Operation Flashpoint is worth a look.
It is a combat simulator that allows the use of the equipment in the game, whether they are civilian (cars, motorcycles) to military, ground or air.
It is also noted for its A.I. ~ the opponent may not be where he was the last time and may not act as he did before ~ each time you play, you cannot predict the enemy.
The cheapest purchase will give you the basic game and the two add ons ~ Operation Flashpoint: Game of the Year.

On a less violent theme I've enjoyed hours with Rails Across America, especially in the multiplayer mode (which is true of the other game too.)

I have no idea what's best...I only know what I like most. Anything in the Myst/Riven, Broken Sword (sadly, only PS1) genre.

I really like all the Sim games. I'm addicted to The Sims right now. I'd like to get the vacation and hot date expension packs eventually. Sim City is always a classic too.

It really depends on what type of games you like.

I've really enjoyed Sid Meiers's Civil War games. My four-year old has watched me play, then set up block of wood in formation as if they're brigades, etc. (I bought him some army men at Wal-Mart after that.)