If you haven't played Dues EX, Thief Two or System Shock 2 you should probably try those.

Played Dues EX and System Shock 2. Havent played Thief. Anymore suggestions?

Thief: Deadly Shadows, Age of Empires, Rise of Nations, Age of Mythology, Unreal Tournament

Rise of Nations definitely.  

Cool ill look into Rise of Nations

Diablo 2 LOD, Median XL-Mod

Total Annihilation.  Excellent RTS.

Yes, if you have not played Rise of Nations, it is worth it. Remember, you can pause the game and roam around and queue things up. A pausable RTS.

Kohan.  Very interesting RTS.  Kohan, Total Annihilation, and Warlords Battlecry 2 are my 3 favorite RTS's.

Dominions 3, on the other hand, is my favorite TBS.

My all time favorite computer RPG is Progress Quest.  It's free!

Star Control II has been remade as freeware for Windows, and it's awesome.

The original Half-Life is also worth playing, if you never have.

When I was younger I used to go down to my local Game and pick up armfulls of budget games. I'd go once a year during the January sales with £40 and walk home with 10-15 old games which I'd spend the following year completing. Many of which I still play. Heres a run down of my favorite (that run well on xp). Too bad Game doesn't do good deals on PC games anymore.

Theif series

The Theif series is really clever, imaginative and very atmospheric. You play as Garret and sneak through mansions, avoiding guards and other taffars, and steal their property. Theif 2: The Metal Age is by far the best in the series, followed by Theif: The Dark Project. The third theif game, Deadly shadows is a bit of a let down compared to the other two but run better on modern systems. The first 2 games in the series run perfectly on XP, for Theif: Dark Project you may need to play with the compatablity modes.  They however do run on vista, but the introductiction breifing you of your objectives and explain the story do not run and ultimatly that ruin the game.

Messiah

Messiah is an awesome little game. You play as a cherub and move around the world by possessing other people and using their bodies you make your way through the Sci-fi setting to kill scientists opening a protal to hell.  It really is a cool little game. My disk copy worked on XP until I upgraded my graphics card. After which it refused to run. I beleive gog are selling copies of the game now modified to run on Vista and Xp although I haven't got round to trying it.

Sacrifice

Another briliant game form Shiny enteriainment. This one is a 3rd person RTS.  You play as a wizard, summon monster (with soul), battle other wizards. It is really very original and very imaginative. This runs perfectly on XP and vista, the CD version has very lax DRM on it. Once you instal it and put in the CD key, you don't need the disk again (except to reinstal it).  It is sold VERY cheaply on amazon's market place, however both steam and Gog also offer a good copy.

Ground Control

Again Another Awesome game. Its an RTS game without basebuilding. You choose your soilders where they land. And then thats it. No more reinforcements, get the job done with what you have. And just to make it that bit more frustrating(in a cool hardcore way) there no save games. Highly recommended, but its sequel (GC2) sucks.

Freespace 2

If you haven't heard of freespace, (IMHO) your not a proper gamer. It a really well done space fighter simulation. Unlike previous games in the genre (X-wing vs tie fighter, Wing commander, ect) capital ships are more than just distant terrain for you to go and bomb. They move and fight and queit obviously have their own intentions. Nothing is cooler than being pushed aside as two of the battleships pound each other into the ground. Runs well on vista and xp and there are some awesome mods which do everything from making the game prettier to adding the campaign from the original game to converting it into Babylon 5.

Dwarf Fortress

Dwarf Fortress isn't techincally an old game. But its free and works on very low end pc. The base game runs with AscII (?) graphics but these can be moded to much more easy to understand graphic. The game is very deep and once you get round the awful graphics its really deep and very fun. You take a bunch of dwarves and lead then to contstruct an underground fortress. The game is a work in progress and more and more functions are planned to be added. If they ever get round to finishing it, it will be one of the best games ever produced

The Exile Series.  Absolutely hilarious fun.  Available from Spiderweb Software I believe as freeware.

Rise of Nations, Deus Ex is a must..., Age of Empires, just a few to name...

Wow, I haven't heard mention of those in quite some time.  Excellent games!

Also an excellent game!

Kohan is a great suggestion.  So many games lack competitive AI.  Kohan is not only a wonderfully crafted game but mess with the settings enough and you will find a set up where you win 50/50 or 60/40 etc whatever you prefer.  This AI is not passive and will attack you on multiple fronts.  Also Kohan had a stand alone expansion.  I recommend you find that over the original Kohan.  And finally don't be swayed by Kohan 2.  The second was not an upgrade.

I would recommend Armageddon Empires by Cryptic Comet.  It isn't that old, but it should run on older machines just fine.