Check to see if Firefox is added to the Startup folder. I see you're running XP so that saves me time :)

• Go to Start > All Programs > Startup

Is Firefox listed there, if so, right click the Startup folder, then click Open. Then delete firefox.exe (Firefox) from there. You can also check to see if Windows adds an entry to msconfig as well

• Go to Start > Run > type msconfig
• Go to the Startup tab and uncheck Firefox if it's there.

For future reference, your startup folder is here:

• C:\Users\<user name>\AppData\Roaming\Microsoft\Windows\Start Menu\Programs\Startup.

Does this also happen if Firefox isn't set as the default browser?

i am posting from a different pc.

the thing was firefox auto started 3 minutes after i installed. it was set as default browser.

so i got worried.

When Firefox starts itself, what page comes up?