Many of the major security software vendors offer some form of an online scanner which allows you to take advantage of their respective anti-malware engine to detect threats without installing the client on your machine. This is a great tool because it allows you to get a "second opinion" on your system if you suspect a threat.
Microsoft now offers their own online scan, dubbed the Microsoft Safety Scanner. This tool, as you can probably guess, utilizes the Security Essentials scanning engine. Considering it gets pretty good overall reviews, if you utilize an alternate security suite this can be a good place to double check your system.
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