When you get your machine cleaned, I recommend Norton Anti-Virus 2000 from Symantec. I downloaded that + Cleansweep + Norton Utilities + Internet Security (all about £30-ish each). The Anti-Virus software reminds me to download updates every 10 days (annual renewal recently for £2.50) and also reminds me to update the rescue disks every 2 weeks or so.
When you install the Anti-virus software, it prompts you to create the rescue disks (8x1.44mb floppies). If you get a virus (because, for example, you turned off the Anti-Virus software and forgot) you boot up from cold using the rescue disks. Following the instructions will allow a clean boot followed by removal and disinfection of the machine.
Although I have never experienced a virus, Norton routinely picks up Trojans and blocks them. Friends tell me that Norton is good...