windows xp help

Please help. I have a brand new dell (july) and ive had issues getting online since august. here is the history. In august, I experienced the machine re-booting on me twice. I installed SP1 for XP. My internet connection speed was spotty. I talked to my ISP (cable modem) and they sent a guy out to check my neighborhoods connection. It checked out ok. service was still spotty. they came out yesterday, gave me a new modem and it looks like it might be my pc. i can get on the internet right after i start up, but shortly after, i get a very very slow page loading if i open a new browser. I watch horse racing through WMP. If i log onto a racetrack right away, that browser will continue to show the live feed at 400k without a glitch, but i can barely get to my homepage when opening a new browser. I used Adaware and quarantined 6 items. its no better. Any thoughts?