Computer lags only when in certain games

[Naurox]

My computer has a serious lag issue, I did multiple virus scans and I've defragged once since the issue began (yesterday actually). This did not help the issue though.
I attempt to play one of 3 or 4 games that have the issue, and my computer starts them up, and then seems to hiccup and lag my video and mouse all at the same time, and it does this every 2-3 seconds.
This namely didn't make sense to me because the games this is occurring on are Starcraft, Diablo II, and Warcraft III. Yet I can play Sins of a Solar Empire, Black and White II, Assasin's Creed, World of Warcraft as well as many other games lag free. Is there any reason it is only happening on these games? And is there a fix for it?
I also reinstalled XP on the primary partition because of the issue, and it did nothing. The system is not overheating, and I have all my needed and correct drivers. (I didn't get any new hardware in the past month or so).

P.S.
Recap: All drivers are up to date. And on a completely fresh install of XP. And already ran CCleaner, and MemTest.

 

[mailpup]

Those older games came out before Windows XP did and are not optimized for it while the newer more demanding games are. I suggest you change the compatibility mode to an operating system prior to Windows XP. You can experiment to see which works best. I recommend that you install any game patches first.

There are two ways to change the compatibility mode. First you can right click on the game icon on the desktop (or on the game .exe file in Windows Explorer), left click on Properties and select the Compatibility tab and go from there.

The other way is to go to Start > All Programs > Accessories > Compatibility Wizard. The advantage of the Wizard is you can try running the game in the various modes within the Wizard to see how the game runs. Once you decide which compatibility mode to use, you select it and exit the Wizard. Both methods work but the first one is slightly less convenient, IMO.