How do you protect my computer from being used by other?

use the classic logon screenin Windows XP Professional the administrator is enabled by default and you can logon to it by pressing CTRL-ALT-DEL AT THE LOGON SCREEN

in Windows XP Home Edition the administrator is restricted during regular operation

u may only logon to the Administrator account in safe mode,

In Windows Vista Home Basic Home Premium Business and Ultimate Windows 7 Home Premium and Professional and Ultimate the administrator account is disabled by default for your protection

please note the following

Windows XP must have SP3 installed

Windows Vista must have SP2

Windows 7 Has not released SP yet A firewall would be a BIG help with this...

If you have Windows XP WITH Service Pack 2 then the firewall is enabled by default.

Windows XP WITHOUT service pack 2 has a firewall, which isn't very good and is NOT enabled by default. Get service pack to and install it now!

Also try...

www.zonelabs.com and download their FREE version of "ZoneAlarm" firewall; it's very good and it's free! Create a passWord protected log-in and do not give the passWord to anyone else.