On many computers, you can just press a key on the keyboard to enter boot device selection (on my current computer it is Esc, on HP laptops it is Esc then choose boot menu, on BenQs it is f2 to enter boot menu, etc.)
In the event that your computer doesn't have such a function:
http://helpdeskgeek.com/how-to/change-boot-order-xp-vista/Its not too difficult, and nothing will go wrong if you mess up the boot settings-you'll just keep booting to the selected device, your computer won't get corrupted or anything like tht.
The only thing I can think of to be careful about is to make sure you are changing the boot settings, though, not the hard drive master/slave settings-on some (old) BIOSes they look horribly similar and if you are in a rush, you will mess up.