Hi again, I thought we were waiting for more info from you? You were booting into Safe Mode weren't you? Well, how did that go? And you were doing a virus scan ? You mentioned about the PC not being on the net, do you mean that it has never been on the net, or it isn't now? Viruses can still propogate through USB sticks, CD/DVDs, floppies etc. This probably isn't a virus, but, it seems a logical step to discount that.
This will almost certainly be a driver clash, but we need more info. Also, sometimes a BIOS reset is necessary as registers can get muddled, one step at a time though. Has he (your son) recently installed anything, harware or software? Have a look in Device Manager (Right click on My Computer, Properties, Hardware, Device Manager) and see if any problems are reported under the Keyboard's section. You could also check the Keyboard control panel within Control Panel and try the troubleshoot option.