Kaspersky

Few anti virus programs detect trojans and viruses better than Kaspersky. Kaspersky's has two online scanners, a file scanner which is geared to check files no bigger than 1 MB making it a great place to check a suspicious looking IM attachment or email. To use it, all you have to do is click on the Browse. A window will appear showing you files in a section of your PC, for example, My Recent Documents. If the file you want is not there, click on one of the other places listed ie. My Computer and search for the file from there. Once you have found the file click on it then press Open in the bottom right hand corner of the window. This moves the file to the Kaspersky scanner interface. You will see that the file is now in the box beside the Browse button on the Kaspersky page. Now press Submit. In about 20 seconds or less, maybe a bit more, you will get a result telling you whether the file is dangerous or not. Remember, Kaspersky's online scanner is designed to scan files no bigger than 1MB. If you don't know how big a file is, run your cursor over the file you want scanned in the window that appears when you hit the Browse button. You will see a little yellow rectangle appear which will tell you the size of the file in MBs (unless you you've got a really big file). If you don't see the yellow rectangle, open Windows Explorer (Start>All Programs>Accessories>Windows Explorer) click on the folder of the file you want scanned and move your cursor over the folder or the particular file in it. The little yellow rectangle will appear telling you how big the file is. You can do it through Program Files as well(Start>My Computer>Program Files).
Once you know the file is less than 1MB head back to Kaspersky's online scanner and follow the procedure described above or the instructions provided at the Kaspersky scanner page.