Needing a little help

I need a little ‘puter help since I am mighty confused and I honestly am overwhelmed by all the choices out there. You see, it’s my computer which is making my life a bit rough for me at the moment and it keeps on bugging and harassing me by giving me these little messages that it is running out of space. I tried doing as much spring cleaning on my drive c: as possible but there is nothing to remove. So, the only two options left for me is either upgrade to a larger hard disk drive or repartition my existing one. Purchasing a new larger hard disk drive is out of the question at the moment since my laptop is only 1 ½ year old and I have an extended warranty on the whole thing except for the laptop battery. The only option left for me it seems is repartitioning my existing hard disk drive. So, the question arises….which software should I use? There are so many choices out there, so I need a little help. The information which I can provide to help you guys out is listed below:
Computer specifications:
RAM: 2 GB
HDD: 20 GB (Drive C :)
80 GB (Drive D :)
TOTAL: 100 GB
O/S: Windows Vista
I have been eyeing the Norton Partition Magic software but am unsure if it can work with the Windows Vista. I was informed that Windows has its own partition utility, but I was not able to increase the size of drive c: for some reason. Yes, using it I was able to decrease the size of drive d:, but I was not able to successfully able to allocate the free space to drive c: for some reason. May be I did not do it correctly. Could someone help me out in this so that I can avoid purchasing repartitioning software? Otherwise, please suggest really good software which is easy on the wallet, safely repartitions the hard disk drive without losing any data and is easy to use. Thanks in advance for any/all help.