weird lag problem in all steam games i have

well i have looked all over the place for my answer with no luck so here is my problem:

i play HL2, HL2 EP1, HL2 EP2, GMOD, COUNTERSTRIKE and PORTAL i love playing all them but what really is annoying me is the fact that i play and it starts to lag but it is sort of like a timed thing because i play perfectly no lag best way to play i love them out of the blue for no reason i start to lag lasts about 10-20 secs as long as a minute once then goes back to playing perfectly at when it was happening in GMOD i thought must have to much stuff loaded in the game and it was slowing it down when tring to read but when i loaded up hl2 today it did it too so thought maybe some one else has had the problem and maybe thay have a solution

my computer specs are:
Acer desktop T180
2.5 G`s of ram
AMD Athlon™ 64 Processor 3800+
2.41 GHz
Physical Address Extension
Microsoft Windows XP SP2 Professional
Graaphics Card Nvidia GeForce 9400 Gt DirectX 9
My Screen is also my tv using VGA wire to connect them

Hope Some here can help me out
would be greatly appreciated

Things i have tried:
Uninstalling and reinstalling
Makinging the games run on DirectX 7-8-9

I dont know that card should play source relatively well, and source was optimized for xp [i think] so, sometimes in a map [on my card] it has to see things and my card doesnt render very well but once it is rendered it doesnt lag in that spot? so i would like a fix aswell

Considering all the things you have tried, it may be something running in the background on your PC. Do you have a virus scan program running in the background?

Sometimes, virus scanners scan every few minutes to check for viruses, which take up a lot of CPU power.

well i thank you all for ur input i found a fix for garrys mod its running with out the momentary lag at lease for now but the other games are still doing it even running on -dxlevel 80 cant go any lower as for the programs i shut down any non needed programs before i start and put my virus scans happens at 6 in the morning that way i dont have to sit and wait for it to finish

FlubberNugget i agree some times its worse depeneding on the map but i generaly play on gm construct or new worrld 2 or flatgrass etc the only other map i play on would be richland night or titanic

but i do thank you two for the great ideas :smiley: