I recently bought a used laptop from a fleamarket, the guy said it works fine (which it does as far as i can tell), but he neglected to tell me the startup password, is there any way to bi-pass it?

Try this:

http://support.microsoft.com/?kbid=321305

I always start the computer in safemode with command prompt, enter the administrative account(people usually aren't smart enough to make a pass for this or change it, so it's typically password free) and open the command prompt. From the command prompt type in "net user (user) (newpass)" without the quotes, and filling in the appropriate spots. Nifty huh?

you can do the same thing by just starting up in safe mode. The administrators account will show up on the login screen. Usually password-free. Then just go to the users screen and remove the password. Alternatively, you could create a whole new account, and delete the pre-existing account, provided you don't care anything about the old data.

how do i start it in safemode exactly?

Mash the F8 key a few times just before Windows starts to boot up. That happens right after you see the "POST" screen disappear.