Automatic Updates kills CPU

Lately I've been noticing that my computer runs EXCRUCIATINGLY slow… so I went into Task Manager to see what was eating up CPU time…. SWEEPSRV.SYS (Sophos Antivirus' on-access scanner) was taking an awful lot of CPU time and svchost.exe was taking up a lot of memory. Accidentally, I killed svchost and not the scanner… and suddenly Sophos started playing nice again. Further trial-and-error checks showed that the Automatic Update service would cause sophos to start eating up over 50% of my CPU cycles! I haven't updated Sophos recently, and this only just started happening, so I'm not sure what could be causing it… it's pretty unnerving. Is it possible that Automatic Updates has been messed up by microsoft in some new update? Or is it possible I've acquired a virus/other malware? Sophos and HJT seem to turn up clean results, and RootKit Revealer gives me some odd registry results that I don't want to touch just yet. Any advice? Logfile of HijackThis v1.99.1 Scan saved at 12:52:13 PM, on 07/01/2007 Platform: Windows XP SP2 (WinNT 5.01.2600) MSIE: Internet Explorer v6.00 SP2 (6.00.2900.2180)