By all means clean up the temp folders, clear the prefetch cache, get rid of the backup folders for windows updates (those starting with a $ under the windows folder), and empty the C:\Windows\SoftwareDistribution\Download folder where updates appear to be cached after download.
Leave any files in C:\windows\installer untouched.
Make sure your policy is set to delete the swap file on shutdown - otherwise this can take up a chunk of space in your image.
If you have System Restore enabled by default, disable it then re-enable it just before imaging - this gets rid of the existing system restore points which can also chew up a bunch of space on your hard disk.