There are quite a few online scanning services but you have to be weary because some of them are scams! They can tell you there's something wrong with your computer just so you'll buy their software to "clean" it. OR, they use this opportunity to gain access to your computer and/or personal information.
Before you consider using an online virus scan ask around (on different forums) to see if anyone else has used them and what ask what did they think of the service. There are a couple of big names out that offer online virus scans but even with them, you should ask others how they liked the service.
Below are two big name scanners but I've never used them so I can't say how good or bad they are...
Trend Micro Housecall BitDefender
Has anyone tried these?