AzharIqbal asked the Other Hardware forum how to see who else is using his computer.
Can everyone in your home or office log on to your PC as you? If so, that's the first thing you need to fix. After all, seeing who's using your PC isn't as important as controlling who has access.
You need to set up a password-protected administrator account--and it should be the only administrator account on your PC. You can do this from Control Panel's User Accounts applet. In Vista or Windows 7, access this by clicking Start, typing user, and selecting User Accounts. In XP, click Start, then Run; type nusrmgr.cpl, and press ENTER.
And make sure to pick a password that close relatives or co-workers would never guess.
If other people have a legitimate right to use the PC, give them their own accounts. But make them Standard accounts (Limited in XP), not administrator ones.
If you're worried that your kids are using the computer too much or at the wrong times, you can control that with a number of child-protection programs. Windows 7 comes with a good one--Parental Controls. The free service Norton Online Family can also help you here.
Even so, you might want to protect your personal digital property in other ways. Keep your sensitive files encrypted--I recommend using the free TrueCrypt for this chore.
And use your browser's private mode if you want to make sure no one else can see where you've been surfing. In Chrome, click the wrench icon in the upper-right corner and select New incognito window. In Internet Explorer, click the tool icon in the upper-right corner and select Safety, InPrivate Browsing. Firefox users should select Tools, then Start Private Browsing.
AzharIqbal specifically asked about spying on other users with a webcam. I know of one program that can secretly record everything in front of your webcam and save the videos to your hard drive. Unfortunately, it will only do this if whoever is on the computer is logged on as you. The program is called Webcam Surveyor, and it costs $30 after a 30-day trial period. If you use it, make sure you put it in stealth mode.
But I have serious reservations both with this program and the whole idea. For one thing, a program that's constantly recording video and saving it to the hard drive can't help but have a negative effect on performance. For another, if someone even begins to suspect what you're doing, he or she can simply put something over the webcam lens.
Read the original forum discussion.
Contributing Editor Lincoln Spector writes about technology and cinema. Email your tech questions to him at email@example.com, or post them to a community of helpful folks on the PCW Answer Line forum. Follow Lincoln on Twitter, or subscribe to the Answer Line newsletter, e-mailed weekly.