Online virus and spyware scanners
It’s a good practice to perform an online scan for viruses or spyware once in a while. The advantage of online scanners is that they always have up to date virus database, hence they can detect even newer infections.
Some of the popular online virus scanners are Panda ActiveScan, TrendMicro HouseCall, BitDefender Online Scan. All of these are ActiveX based and hence require Internet Explorer to run. But TrendMicro HouseCall has a version of scanner which can be run on Opera, FireFox, NetScape based browsers. It is available here.
Ewido Online Scanner, TrendMicro Spyware Scan are some of the good spyware/adware scanners available. They have good detection and removal capability.
If a specific file is to be scanned to check whether it’s "good" or "bad", then Jotti’s Malware Scan can be used. This site uses multiple AntiVirus engines to scan the file so reliable results can be obtained about unknown files. Similar service is offered by Kaspersky File Scanner.