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To make this simple and pain free, allow me to make an observation. Un less you hard drive is very small, or you have a special need, there is little reason to use just on large partition. IMHO the decision point is around 80 GB. If the drive is over 80 GB you can divide it into two partitions. Over 160 GB maybe three partition.s At least one partition has to be a primary to hold the boot loader.
In both Vista and Windows 7 you can adjust the size of a partition. But with some limitations. Some free partitions managers allow you to delete, create and size partitions. At least one allows merging two partitions with no data loss. I have not tested this and would be reluctant to recommend it.
Do you have a way of doing a backup of your personal files? Changing partition size can led to a lose of data in some cases. Not very often.
Deleting a partition could make your system unable to boot. The boot loader has to be on one of the partitions. It is not always clear which is the active partition.
My recommendation is to backup anything very important, like family photos and business letters. Afterwards use a partition manager to reside the partitions to make best use of your hard drive.
Forgot to mention, Windows 7 has tools to let you data on another partition yet still have the file system you like. What i mean is that a folder on one drive can really be a reverence to another drive.
Typical of this is moving MyDocuments to another location,even another partition or even another drive. I strongly recommend this. It makes it unnecessary nto merge partitions. I believe having separate partitions used this way gives a better level of reliability.
The link below is from Microsoft.
How to Change the Default Location of the My Documents FolderIs this any help? Please respond if you have questions about this recommendation or anything else.