There are one or two things you can try first.
One possibility is that your 'C' drive has filled up with files and there is very little space left. Go to START menu and choose 'My Computer'. Then Right-Click on your 'C' Drive and from the menu that appears, choose 'Properties'. That should show you as a pie chart how much of your storage space you have used up. If it is showing as more than 90% full, that could be your problem.
If that is the case then there is really only one solution delete something that takes up a lot of space such as music or video files. Before deleting it of course, you will probably want to move it somewhere else. Many computers these days are coming woth a partitioned hard-drive with both a 'C' and 'D' partition. They don't normally save data to the extra partitionof their own accord so the 'D' drive stays virtually empty. Drag your datafiles to that drive, verify that they can still be read and then delete them from your 'C' drive.
Another possibility is that your folder of 'Temporary Internet files' has grown huge, so try deleting that as well.