Probably the brontok worm. Though it could be adware trying to push a cleaner on you. Your IE may have been hijacked.
Use the AV lord recommended, do a full scan and then run sbybot and adaware sweeps.
And if you don't have them get a firewall and AV up.
Zonealarm and AVG or the AOL Kapersky knock off and keep them up to date with automatic updates.
ANd never open junk emails. 1 in 5 are viruses
AVG typically blocks 3-5 infection attempts per week on my machine.