Hi Party, im assuming you are using Windows XP?

short term options -
1. change the location of your 'My Documents' folder to the 'D' drive -
heres how - http://support.microsoft.com/kb/310147

2. change the location of the Page File to the 'D' drive - http://support.microsoft.com/kb/307886

3. uninstall any unused software - http://support.microsoft.com/kb/307895

there are a few other cleanup routines you could run but i believe others have already suggested those previously

long term - a 40gig drive is simply too small these days Party - so backup everything important to cd/dvd, purchase a new hard drive, min 200gig, & reinstall Windows to the new drive. You could leave the old drive in your pc so as to copy over all necessary documents etc to the new drive, but that drive would need to be set as a 'slave'

hope ive been of some help
stugs