not sure that you have a virus. at first check this:
"If you are receiving error 80048820 when trying to sign into Windows Live Messenger you should also get an extended error code. If this extended error code is 80048412 the problem often occurs because the time and date settings on the computer are incorrect."
More info
i mean you have any antivirus sw. if haven't, that's big mistake
perhaps you can use
trendmicro online scanner or
One Care Live to scan viruses/trojans on your pc.
in the last resort:
reinstall your vista, and install an antivirus sw before you connect to the internet.
/ 
