Download crashes

I run XP SP2, wireless connecction via Belkin adapter/extender to Belkin
router.
After having problems with the connection, it seems to be sorted after
repairing Windows and re-installing Belkin software. I also run Norton
Systemworks
Now downloads, incl Norton and MS updates, start off fine and then crash
after quickly decreasing download times.
I have tried to repair XP, Belkin re-install , and new network connections.
I have tried the Firewall/Anti-virus on/off. I have put in the IP address
manually as well as auto. I have tried Dial-a-Fix, System Mechanic, and a
Winsock repair.
Emails seem to download ok but webpages and updates slow the system to a
crash. I did manage up update Ad-Aware, which found 3 minor bits, but nothing
serious. The same with Defender tho I was unable to update its definitions.
Oddly, I found that Ad-Aware, Defender and even Wireless Services disabled
themselves, tho I could put them back to Auto.
What do I have left to do other that an XP clean re-install? I have wasted
so much time.
Thx

Mike