you don't really need worry
XP and Vista both prefer to have anout 20-25% free disc space + approx 1.5x your system ram (for system files)
first let's make a point
there is a difference between storage and memory .... storage is what you use the hard disc for
MEMORY is what the system uses to do it's job.
When you deal with graphics or spreadsheets ... all the calculations and communications between mouse, keyboard, screen etc are stored in the memory - and then overwritten when the job is done.
The program you are using also likes to live in memory.
when all this stuff needs more space (memory) than you actually have fitted to the machine - windows starts to write the information to a pagefile on the hard disc.
the system "knows how much disc space to reserve (on average) .... so if you do something different that needs more - windows will expand your - etc etc
CC cleaner is a good idea (once a month should be often enough!!)
it sounds like you could do with (either/and)
fitting more RAM
http://www.crucial.com/uk/index.aspx?cpe=CHAWK uk
backing up and removing