Just before we begin a partition is a part of a hard drive. If you have one partition then your hard drive is whole. but if you have two then that means your hard drive is split in two (not necesserily halves). This decreases performance and you should only have one.
OK here are step by step Instructions.
1. Put your vista disc in the drive
2. restart your PC and as it loads up tap F12 (where you would usually go into bios or the screen with your motherboard logo on it)
3. select CD-ROM
4. press enter until you see the screen "Windows is loading files
5. get a frosty beer while you wait
6. select your language settings
7. next
8. Install now
9. Put your product key in
10. next
11. next
12. Custom (advanced)
13. click "drive options (advanced)" (at the bottom)
14. Go over all partitions and delete them all (make sure there are no periferaly plugged in eg. external hard drives, pen drives.)
15. with ALL unallocated space click on NEW
16. not sure if it gives you the option here but select FORMAT instead of QUICK FORMAT on the screen that may or may not come up. if it doesn't don't worry.
17.When the partition is created click on it
18.Click next
19.Let it install (pretty simple from here)