The remaining space has been left unpartitioned. The easiest option to "unlocking" the rest of your hard drive space is with a partition manager of your choice. Personally, I use a copy of Partition Magic whose license I've obtained through work. This program does cost money, but I'm sure there are many free programs that can do what it does available through a simple google search.
Hope this helps!
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I see you updated your question. Have you tried using a partition manager yet? Also, I forget which service pack it is, I believe its 1, but it addressed some common hard drive issues. Not similar to yours, but may be related none-the-less. What SP are you using?
Here is a free Partition Manager that is extremely similar to Partition Magic: http://www.download.com/Easeus-Partition-Manager/3000-2248_4-10863346.html
Let me know if this helps.
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SP1 is the one you need. Please run this tool below, restart, and the use the partition manager software to see if you see some "Unallocated space"
http://www.48bitlba.com/enablebiglbatool.htm