How to add more space to my c drive using Vista

You have three routes:
1) Buy Partition Magic, install it and re-partition the disc. If you have a way to do it, back up all your data first.
2) Backup your data. Download the free, Open Source partition editor, GParted
Burn it to CD, following the instructions included with it. Boot the CD and change the partition sizes.
2) Back up all your data and re-install Windows from scratch. You may have to create the CD/DVD for this from the disc images already on the PC. There should be a utility already installed to do that if this is required.

Whilst doing so, you will usually (but not always!) be asked which disc partition you want to install Windows into.At this point, you can delete all existing partitions and create new ones (or a single large one) of the size(s) you want. When all is finished, restore your data.