Aris Cristian doesn't mind friends using his PC, but he worries that they might damage Windows or his files.
Generosity is a virtue, but so is taking precautionary measures. If a visiting friend needs larger-than-mobile Internet access while at your home, you don't want to deny it. But neither do you want new browser settings, a malware infection, or a formatted hard drive.
[Email your tech questions to firstname.lastname@example.org.]
The first thing you need to do is password-protect your own user account. That way, only you can log onto your PC as yourself.
You can do this in Control Panel's User Accounts applet. In Windows 7, click Start, type user accounts, and select User Accounts under Control Panel. In Windows 8's Start screen, type user accounts, select Settings, then User Accounts. Everything should be clear from there.
Since you don't want to give anyone else your password, activate the Guest account. This allows your friends access to Windows without a password, but with very limited capabilities. They won't be able to change settings, install software, access your files, or format your hard drive.
You can activate the Guest account from the User Accounts applet. Click Manage another account, click Guest, then click the Turn On button. When you boot your PC, you'll get options to log on as yourself or as a guest.
If you have some very private files that you don't want others to see, keep them encrypted. I recommend downloading and installing Truecrypt (it's free), and using it to set up a volume, also called a vault or an encrypted file container. A Truecrypt volume, when unopened, is a large file containing gibberish. Opened with the password, it's another partition in which you can store, read, and write files.