Look for and research any process connecting to the Internet you don't recognize. This part of the search can take a long time and require more investigation. Usually, you'll find lots of legitimate programs connecting to the Internet; I seldom disconnect any legitimate program. Who knows what you'll break? I keep an eye out for strange programs I don't recognize connecting to suspicious-looking websites. You can often use the autostart programs to remove offending software.
If you find something suspicious with any of these steps, disable it from automatically running. As a last-ditch effort, I'll boot into Safe Boot mode (F5 or F8) or from another OS copy, then rename the suspicious file so that its autoloading program can't find it. If the file is needed and legitimate, you can rename it and your system will function normally again.
Last, but not least, try rerunning your antimalware program. Sometimes malware in memory prevents antimalware software from successfully identifying it; when you prevent malware from loading into memory, the antimalware software may do a better job.
I've been cleaning PCs like this for over two decades. Normally, I'll find one or two malware programs and manually remove them from the PC. Then I'll rerun the antimalware scanner in quick-scan mode, followed by a complete scan. Usually, the antimalware program finds one or two (or 200) hidden malware programs I didn't pin down. Either way, you should have a significantly cleaner PC.
None of the preceding advice is perfect. Malware is often designed to hide from prying eyes. If you think your computer is still infected after all of the above measures, start fresh: Format and reinstall. Nothing gives peace of mind like knowing for sure that you're system isn't infected. Plus, your computer will run faster and have more disk space -- three benefits for the price of one suspicion.
To see how to keep your new install clean, read "The 5 cyber attacks you're most likely to face." Follow the countermeasures there and you'll vastly reduce the chance you'll need to scour your system again anytime soon.
This story, "How the pros sniff out a malware infection," was originally published at InfoWorld.com. Keep up on the latest developments in network security and read more of Roger Grimes' Security Adviser blog at InfoWorld.com. For the latest business technology news, follow InfoWorld.com on Twitter.