I assume you mean you reinstalled Windows? That wasn't really necessary, but now you've done that, your drivers probably aren't all installed. Your video driver will be one, that's why you can't change the resolution.
Go to Device Manager - Right-click My Computer > Properties > Hardware tab > Device Manager.
What devices have a yellow question mark next to them?
Do you have onboard graphics?
-------------------- OK OK I can't go any faster! aaaaaaaaaaaagh!