Malware Scene Investigator is a portable free tool which quickly scans your PC for security problems. It can't then fix them - you'll get warnings only - but that's still enough to be very helpful.
The program is very small (a 482KB executable), and extremely fast, checking our test PC in just a few seconds. A tabbed interface then displays the results in two forms: clicking Report provides a quick summary, while the Detailed Log tab gives you a more in-depth view.
Despite its small size and extreme speed, Malware Scene Investigator tries to cover a wide range of issues. It can highlight HOSTS file manipulation, unknown drivers, and dubious proxy settings. It looks for unusual disk partitions, Registry modifications or startup programs. You'll be warned of executable files in your temporary folders (a common route for malware), and the report includes general information about your PC's state (open network connections, running processes, scheduled tasks, recently created \Windows\System32 files, and more).
We tested this out on a test PC, and the program did generally quite well. It highlighted some active third-party drivers, for instance; picked out an unusual Windows service setting; even warned us about a suspicious startup entry which we had already become concerned about.
Checking for outdated software was less successful, though, at least in our case. The program claimed that we had an outdated version of Flash installed, but this wasn't true: it just hadn't identified our installed Flash version correctly.
Malware Scene Investigator isn't for novices, then. And you can't rely 100% on everything it says: the program can only give you pointers as to what you should investigate next.
But, if you are worried a PC has been infected by malware - or just want a general security check - then the program is worth a try, and it certainly deserves a place in your security toolkit.