Allocating unpartitioned space

So, I'm on this new computer and apparently the guy who installed it was a complete moron : out of the 500GB hard drive, 400GB are unallocated. Not very useful. Is there any way to allocate all that space that doesn't involve getting some weird program?
(I was pretty sure I could do it from the WinXP installation CD, but it only reinstalled the system, froze a bit, and now everything seems to be more or less exactly the same as it was earlier)

Thanks in advance