Program Files and the Common Files folder are part of the ''System'' and best left where they are (default location). You may be able to change your partitions to more suitable sizes (e.g. increase the C: partition) by the use of, e.g. Partition Magic ( try putting changing partition size into Google). If you choose to do that, back up your data files which you cannot afford to lose.
I just built a new computer and installed Windows XP Pro with SP2 on it. When I set up my hard drive and OS, I partitioned my lone hard drive into three partitions: 10 Gb C: drive for Windows, drivers, etc., and a pair of 111 Gb drives for things like program installs. I noticed that my C: drive is filling up rather rapidly, and I discovered that Program Files\Common Files is taking up a considerable amount of space. I would like to move this folder onto one of my bigger partitions, but I'm not sure how to do it without screwing up all my programs. Any direction on this matter (that doesn't involve re-partitioning and starting from scratch) would be appreciated. Thanks!