Here is a quote from Microsoft regarding the performance impacts of NTFS compression:
"NTFS compression might cause performance degradation because a compressed NTFS file is decompressed, copied, and then recompressed as a new file, even when copied on the same computer. Similarly, on network transfers, the file is decompressed, which affects bandwidth as well as speed."
Compressing a whole drive is a big mistake. If you were that desperate for space, you needed to move things or get a new drive.
How did you perform the whole drive compression?
You can do it from a command-line with UBCD:
COMPACT: Displays or alters the compression of files on NTFS partitions.
COMPACT [/C | /U] [/S[:dir]] [/A] [/i] [/F] [/Q] [filename [...]]
/C Compresses the specified files. Directories will be marked so that files added afterward will be compressed.
/U Uncompresses the specified files. Directories will be marked so that files added afterward will not be compressed.
/S Performs the specified operation on files in the given directory and all subdirectories. Default "dir" is the current directory.
/A Displays files with the hidden or system attributes. These files are omitted by default.
/I Continues performing the specified operation even after errors have occurred. By default, COMPACT stops when an error is encountered.
/F Forces the compress operation on all specified files, even those which are already compressed. Already-compressed files are skipped by default.
/Q Reports only the most essential information.
filename Specifies a pattern, file, or directory.
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The only way I can think to undo this is to uncompress all the files from another installation or from a bootable OS like
UBCD4Win. You could see if the "compact" command is available from the Vista recovery environment. I am not sure that UBCD4Win has a program or functionality to uncompress a drive, but you can load:
Servant Salamander can undo the compression, though I'm not sure about the free version 1.52.
NTFS Ratio is a free context menu add-on that will compress/uncompress on NTFS volumes.
And NTFS File Decompressor can undo it, but it costs money.