Did you ever run ChkDsk /r on this drive?
Have you looked at the System Restore options when you ran Disk Cleanup. This may be consuming your free space. You can look at the "More Options" Tab for further cleanup options.
"System Restore and Shadow Copies. Prompts you to delete all but the most recent restore point on the disk.
System Restore uses restore points to return your system files to an earlier point in time. If your computer is running normally, you can save disk space by deleting the earlier restore points.
In some editions of Windows Vista, restore points can include previous versions of files, known as shadow copies, and backup images created with Windows Complete PC Backup. This information will also be deleted. For more information about System Restore, search Windows Help and Support for "system restore."
Apart from Basic, the System Requirements are for a 40GB with 15GB of free space.
System requirements for Windows Vista
Have a look at the Extend parameter in the Diskpart tool.
New, Free, Graphical Partitioning Tool
Resize Partition, (try the free trial).
BootIt NG
When you purchase a new hard drive, the drive manufacturer will have free software available to copy your old drive to the newly purchased drive.
Any resizing or partition work is risky and preserving valuable data via backups is necessary.
AntiVirus on vista
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