What is the exact value for the remaining space ?
31% of a 40 gb hard drive is a lot different than 31% of a 5 gb hard drive
I am guessing the hard drive is ok unless it's sverely fragmented
I don't think more memory will help if the hard drive has issues
I would try to find out why is the hard drive crunching so much
Check to see if you have some program running in the background like an update or something like that.
I would also check the event viewer to see if there are any errors