what is virtual memory?

Virtual memory in it's simplest explanation is an extension of your physical memory to your harddisk, virtual memory uses disk space. In your case try changing the setting of paging file into "let system manage size" option under virtual memory setting to be safe and make sure you still have enough disk space on your harddrive.

To say it simply... You have a certain amount of RAM memory in your computer. Now once you exceed that amount of memory, your system would automatically go to your hard disk to create a memory space for your program/s to run. It is because your programs or files occupy a certain amount of memory. The allocated memory in your hard disk is called virtual memory. Now, don't worry. Since it's virtual the allocated space is only temporary. So be sure clear up your hard disk every now and then. I hope this is much understandable.^^

Virtual Memory is simply the operating system using some amount of disk space as if it were real memory. Exactly how virtual memory is implemented is complex and well beyond what I'd want to present here. But in an over-simplified nutshell it works like this: You run programs that need memory. The operating system takes care of tracking which program is using what portions of memory, and allocating each program the amount of memory it needs. Those programs will need more memory as they do their jobs. Opening a large document may cause your word processor to request additional memory from the operating system in order to hold the document. If there isn't enough memory available to satisfy a request, the operating system may decide that another program's needs are less "important". Some of that program's memory will be freed, first by writing the contents to disk (the memory is "swapped out"), and then allocated to the program making the request. Later when the program whose memory was swapped out needs it back, that memory can be "swapped in" by reading it back from disk. This might cause memory from another program to be swapped out to make room.