I take it your op system is? on the 20G drive and you always install things to their default c:\program files\xxxx
Try installing new progs to the d:\? 80G drive.
Set the virtual memory onto the 80G drive by control panel - system -advanced -performance options -settings -advanced - virtual memory -change - click on new drive click system manged size click ok.
Empty temp internet files and all temp folders and recycle bin.