Firstly, we need to be clear. The memory I am talking about is called RAM, not the 'memory' which exists on your hard disk to store programs and files in. They are measured in the same way but RAM is more expensive and usually smaller, for instance 4 gigabytes (Gb - approx 4 billion bytes) whereas a hard disk might be 240Gb or larger.
The point here is that a hard disk although cheaper is much slower because it contains moving parts, literally solid disks of data being read by a moving head. RAM is electronic and very fast.
When you run programs on your PC, they will require a certain amount of program memory and this will be allocated from your PC RAM. However, if you do not have enough RAM (if you are already running too many programs and/or you don't have much RAM in the first place) then the computer will still allocate the memory but it will put it on the hard disk instead - a bit like putting your possessions into storage when you run out of room in your house.
The data can be moved into and out of this 'virtual memory' but like the storage unit, it takes much longer to send it and get it back. This makes the computer slower because programs wait for the memory contents to come back before they can carry on. You can tell this is happening because the hard disk will spend lots of time spinning.
As time goes on, programs require more and more memory, partly because they do more things and partly because people tend not to worry about memory requirements when writing software.
The easy answer when this happens is simply to buy more memory for your PC. You can either search a site like for what memory your PC takes or take the model number and make to a shop where they can check for you. Most memory is fairly standard and you should be able to get at least 4 Gb depending on whether your computer can access this much (after a while, there are too many spare rooms in the house and you have to go back to using storage). For most people using non-specialised software, this is more than enough memory to get good performance.
1Gb will cost around £15/$25 in the UK so it is not much money to improve your PC.
If your PC is ancient, you are probably better off buying a newer basic model for a couple of hundred pounds then all the components will be improved.