The "in use" light on your PC is I think for your hard disk activity.
This is not the same as the amount of memory used on your PC which is RAM (real) memory.
So your RAM memory may have space, but for some reason your hard disk is thrashing away.
This may be because an anti virus or other security program is running (or if you have more than one secuity program on your PC they could ALL be running).
Or it may be that the PC is indexing all the files on your PC hard disk.
Or it may be that your hard disk is nearly full up and it is struggling to find space to fit all the files it needs. Maybe Windows is trying to create a temporary file and there is no room.
So:
Check your hard disk and make sure it is not nearly full. Clear out some files if it is, or back them up to an external hard drive.
Check which security software you have on your PC and make sure you dont have more than one, causing conflicts.
Also tell us what operating system you are using (Windows XP, Windows Vista etc).
Also tell us what computer you are using and what CPU you have and what size hard disk you have.