By far, malware problems are the most frequent support case out there. You hunt and remove the buggers on regular maintenance sessions. New customers would often call you, asking to fix a machine that’s strangely behaving, the screen is locking up, etc.
Naturally, once the problem has been identified as malware, the first thing you’d want to do is scan your new customer’s machine. By far, your best bet would be a LiveCD scan but that can hardly be arranged remotely. Still you can send your customer a link to download an up-to-date image, instruct them to burn it and boot from the CD/DVD.
Alternatively, you can remote into your customer’s PC and download a free scanner. Then run a scaThen run a scan and remove as many intruders as possible. Then proceed to check if there actually was an antivirus installed and running, if it was up-to-date and overall a trusted and efficient one. You may want to talk your customer into buying a good one instead of regularly paying you for cleanups. It’s entirely up to you, of course:)
You may want to instruct your customer to wear an antivirus when going online, especially when visiting certain sort sites and/or downloading software from not well-trusted sources. Note that illegal software modifications may lure the users into turning down the protection... Okay, we all know that piracy is bad and there’s not much to add:)
Not many viruses exist for Macs, mostly there are rootkits. You might want to run a LiveCD that reads Mac partitions and detects Mac-specific malware. Also, there are antivirus products for Macs.
Naturally, a lot of products exist here. A free one supplied with the OS (Microsoft Security Essentials) is actually quite good.
Linux users are mostly targeted by rootkits. This way, anti-rootkit tools are your best bet here. Again, you might want to boot from a LiveCD.