Mnay thanks for the (many) recommendations for AVG.
Over the last two days it has picked-up 10 attacks from the e-mail virus Sobig-F, and dealt with them all effectively.
One question though, the files it trapped were in C:\WINDOWS\TEMP, and were all .PIF files, which could be the old MS-DOS shortcut "Program Information File". If so, has the virus executable been left anywhere else?
I've scanned after each group of attacks, and AVG says the PC is clean.
Any one care to elaborate, or am I being paranoid.