new harddrive, installed xp and still can't access certain sites

Ive scoured malware sites etc.
had winxp on this computer and got some nasty virus, decided to reformat and reinstall, of course the barcode on the side of the computer needs the actual disk so got a copy of xp sp1 and installed. still problems (computer shutting down, internet disconnecting). Good time for an upgrade, got a new 160g hard drive, installed, put on the winxp and all good.
log onto internet, download microsoft updates.... and then can't access microsoft (or avg) again....
tried the ping thing (doesn't connect to these sites either)
did the flushdn thing... no difference.
have reinstalled the os and reformated about 6 times... i can access microsoft for about the first 10 minutes then no more. Tried going to avg first but of course after downloading it tells me i have to update xp first, which i then cannot do as can't get onto microsoft...
I don't think its a malware problem (unless it is being sent to my isp if this is possible). The xp is genuine copy. The only thing i'm not sure on is dns(?) settings.... or when i install the modem it asks if i require 64 bit (which i dont think i do).
help... this is a real problem as i want to be able to do online banking etc.
When avg did install (on one occassion when i got the avg off my memory stick and managed to get win updates) it cannot hook up to get updates.
Thats about it.
am new to forum but have read here for awhile.
hopefully someone can help and then i can move onto getting sound....