Windows works not proper, to slow with strange things

hi, i upgraded my PC from windows 98 to XP Proffesonial and i have new
problems that i have never encounter in wind 98.
when i connect to the internet and then play a game, it play very slow and
the sound is corrupt, for exmaple when i play Quake III when the internet
connection is turned on it very laggs.
and when i play without internet connection it has a strange thins, that
every second it freezes the game for a second and contiune to play then it
freezes again for second and contune to play then again... it countious about
one minute and then it works ok, on windows 98 it works very well, and all
the other games.
and when i play songs on Winamp 3 or Media Player it played very slow with
corrupt voices, can any one help me pleasE?
my system info is:
Intel Penitum III AT 500 MHZ, 256 MB, Creative sound blaster 16, Nvidia
Geforce 2 mx 400 32 MB, 20 GB of Hard Disk, 100 MHZ Bus Memory, Directx 9c,
Motherboard SIS 5600, Windows xp With SP2, please help me!!!