Dave...get yourself some AV sofware ASAP...seriously!!!
There are free ones about (google will find some easy enough) or it really is buying some (unless you're a dodgy unscrupulous thieving bastard like me in which case nudge nudge kazaa lite wink wink) but you really really do need some NOW!!!!!
Most modern virii don't just




about with your PC but will send themselves to every other bugger's oc who's on your list......and you won't even know about it.
My Norton stoped three Trojans this morning alone, two from a 'special interest' site

and another from an email from a mate who I now know has a virus on his system sending out stuff.
IMO Virus software should be the absolute first thing you put on a pc....both foe protection and because it runs at a very low level and can screw stuff up if you put it on as an afterthought.
For decent immediate cover, you can usually get a 30 day trial evaluation of Norton or McAfee ....that'll give you some time (or obtain an unlock code

to buy some.....but don't delay mate!