You can delete old files applicable to SP1 and SP2. Just search "clean up SP1 vista" and same for SP2. These tools, which are built in to Vista are run as a command line prompt and WILL make the service packs permanent i.e. you cannot uninstall them or use system restore to go back prior to there install.
I've run these tools and it frees up considerable HDD space.

1. VISTA SP1 INSTALLED
There is a ‘run-once’ program in SP1 that will immediately clean up all the components that were replaced in the Winsxs folder when SP1 was installed. NOTE that once you run the program you will no longer be able to remove SP1 should any problems occur, so make absolutely certain your computer is behaving itself before running it. Firstly, check the size of the folder by right-clicking it, choose Properties and make a note of its size and the number of files and folders. To run the program, open Windows Explorer and navigate to C:\Windows\System32, look for the file VSP1CLN.EXE (the capitals are for clarity only), right-click it, select Run As Administrator then reply Y to the prompt. Now check its size again. You will be pleasantly surprised by how much disk space you have reclaimed.

2. VISTA SP2 INSTALLED
If you have SP2 installed and are happy with its performance, you can use the Microsoft program compcln to remove files that SP2 has replaced. NOTE that once this program has been run you will not be able to remove SP2. To determine the disk space saved, have a look at the Properties (numbers and sizes of folders and files) in the \Windows folder both before and after running the program.

To run it, click the Windows Orb (Start) > All Programs > Accessories, right-click Command Prompt then ‘Run as Administrator’. Copy & Paste or type %windir%\system32\compcln.exe and press Enter. If you Copy & Paste the command, use mouse right-click to Paste it into the prompt.

If you want to know which files have been removed, Copy & Paste and run the following dir(ectory) commands, the first one before running compcln and the second after running it:
dir "%windir%\" /b/s/a:-d>"%userprofile%\desktop\FilesBefore.txt"
dir "%windir%\" /b/s/a:-d>"%userprofile%\desktop\FilesAfter.txt"

Just in case it's not clear on the Vista SP2 cleanup, I should have said double-click FilesBefore.txt then FilesAfter.txt on the Desktop to display the differences.