The machine is an ex-corporate Toshiba Portege R300 running XP Pro.

Symptoms are:

svchost.exe running at 100% CPU for long periods (hours).
In Network Connection a shared Internet Connection referring to skype has appeared which can't be deleted.
Intermittent internet access over wireless connection appearing to disable router (requiring reset), fine over wired connection.

Recent activities:

Upgrade to XP SP3 clashed with Norton 360. SP3 removed and ultimately Norton leaving a SP3 bug where Network Connections and Devices in Hardware Manager "disappear".
SP3 reapplied, MS fix for bug applied (note this involved deleting registry keys - backup was taken). AVG and Windows Defender installed.

Nothing has reported malware (including SpyBot S&D).

Any pointers gratefully received!

Do note that this was Symantec's fault, and not Microsoft's (this time) - though MS was the first to come out with a fix here. Note it does NOT involve deleting any keys from the registry so I am not sure what you did.

Have you tried a system restore back to before the problem started?

Thanks for your prompt response.

I experienced the problem ahead of the MS patch and their solution at the time involved deleting registry entries beginning with "$%&'(" (without the quotes). This did solve the problem and I don't *think* it is the root cause of my problems but thought I should mention it for completeness.

Will try your suggestion of a system restore.