Windows tries to put all the programs you are running (including itself) into real memory (RAM). If you have enough real memory then normally it can manage.
But if your PC has not got enough real memory (or you start a lot of programs) your real memory can fill up, and Windows needs to do something about it.
What it does is create an area of your hard disk called Virtual Memory and then copies some data from your REAL memory to your VIRTUAL memory.
When it needs it again it copies it back from VIRTUAL memory back into REAL memory. It will continue swapping data if REAL memory is full up.
This swaping process (in computer terms) is a very slow process and can really slow your computer down.
When Windows needs the area of hard disk (virtual memory) it "grabs" as much as it needs, but if your hard disk is nearly full then it has not got enough space, and so you may start getting these errors.
Two things you can do
1) Simple solution, free up some space on your hard disk
2) Add some more REAL memory (or RAM)
In the long term you probably need to add more real memory.