You need to provide more information about your PC. System specs, drivers etc. Copy and paste a dxdiag
To get this do the following -
click RUN (hit enter)
a small box appears. delete anything in there and then type
dxdiag (as i have typed it. no spaces, all lower case) Hit enter
It might ask you to digitally assign stuff. click yes (doesnt matter if it doesnt)
This can take a few minutes to carry out the next task, so be patient.
A box will appear with tabs etc. If you copy and paste the first tab info into this form, then click on DISPLAY and copy those details as well.
Also, how many icons do you have open by your clock? The more of these open, the less memory you have left to play with