Thank you everyone for your replies.

I have Windows 98 as the OS. I have enabled e-mail scanning and it is scanning the new mails. what i wanted was the solution for the existing mails. I'll try out the online scans proposed by Airhead.
Since my NAV dosent show any virus infections, i dont know whether i should download all the antivirus tools and run them. The problem with me is i'm using a slow dialup link to connect to internet. so i cannot be online for long.

Thanks once more for support. Please let me know any furthet details.