Although it doesn't look from what you say that you should have taken up nearly 10Gb, XP itself plus Office will take up a little over 2Gb.
Try moving your pagefile to the second drive and see if that gets rid of the message at boot up.
Do you have Partition Magic? If so, what does it show.
If not, download Disk Size Manager click here It is a shareware utility that will allow you to take a look in some detail at what is on the C: drive and may pinpoint the problem.