I did the same thing about 30 minutes ago. My hd had two partitions and I just wanted one. I used a free program
called Easeus from download.com
Make sure that you move the stuff that you want to keep over to the partition that you want to keep, then make sure to
delete the unwanted partition, then you can use the slider at the top of the screen to increase the main partition size.
After a reboot the program will run both tasks. It took mine about 30 or 45 minutes to complete. Then after windows fully booted I rebooted again. Tada, now everything is all good.
Easeus did suggest that I backup everything first, but I didn't and luckily everything worked great. I use windows 7 btw.