need help with lag spikes

hi everyone

i keep getting lag spikes throughout my games ., thing is it varies from game to game .. ET is unplayable .. Cod4 has lag spikes but seem to not cause many problems playing in wars etc etc.. and COD5 is just Taking the fucking piss ..

i've installed new drivers, gave a ring to my internet providers who werent much use , restart my internet before each game , shut down all background programs but nothing seems to effect it ..

here are my system specs .

AMD Athlon(tm) 64X2 Dual core Processor 5000+
2 gigs of RAM
ATI Radeon 2600 HD PRO

my internet provider is Virgin ..

thanks for anyone with insight into my problem .