Could be your harddisk is getting full.
You need minimum 15% free space for a PC to function properly.
In Explorer, rightclick your C-drive and select Properties.
That shows you how much free space is left.
If not enough (15%) space, you either need to do as Rik said (above) or buy an extra harddisk.
If there is enough space:
Click on Start/Settings/Control Panel and double-click System.
Go to the Advanced tab and click on Settings under Performance.
On the next screen click Avanced tab again, and click on Change at the right bottom.
Click on the button Custom Size.
Set initial Size to 768
Set Maximum Size to 1536
Now click on the Set button, then OK it till you are out of System settings.
You may need to reboot.