"What is the c:\windows\winsxs\manifestcache\6.0.6001.18000_001c50b5_blobs.bin file and why is it taking up 156Mb on my hard drive? Do I really need a file this big?"
That file is used by the Windows Update mechanisms in Vista. It acts like a cache and is capped at a certain size, but it's safe to delete. The next time you turn on or off Windows optional features or take updates from Windows Update it'll reappear. The file will be recreated as necessary to help the servicing system do its work. Maybe in the future it'll be added to the disk-cleaup wizard, but it noticably increases the performance of servicing operations while it's there.
Of course, actually deleting the file is left as an excercise to the reader.