There are 2 type of memory - physical and virtual. Easy and free way - increase virtual memory. right click on "my computer" - "properties "and searh for "perfomance" button. Here you could change your virtual memory size. To increase physical memory you should read your motherboard manual to find what type of memory you could install. P.s. In most of descktop windows OS there is no sense of installing more than 3.5 Gb physical memory.
If you are speaking about ram, install additional modules, they are inexpensive now. The free way: remove unneeded background programs that you don't need and are consuming memory resources.
In order to increase your computer memory you have to do two tasks first increase you RAM When you consider increasing the RAM on your computer, keep in mind that you are adding a physical piece of hardware to your computer and second Alter Your Virtual Memory Settings To alter the virtual memory settings: Right-click the My Computer icon on your desktop, and then click Properties. Click the Performance tab. Click Virtual Memory, and then click Let me specify my own virtual memory settings. Click the hard disk that you want to use for virtual memory, and then select the minimum and maximum amounts of virtual memory. Click OK, click OK, and then restart your computer. now its done
No reason given as to why you need more memory - Have you installed a new program thats insanely memory hungry or did you buy a cheap deal with the minimum memory which goes to pot when you actually install a few programs. Anyway as mentioned multiple times buy more or greater capacity memory modules and install them. However I'd do a full virus scan because if everything was ok and it's just suddenly slowed down to a crawl with all the memory being used then you could have a virus.
I agree with all the above post. By adding the physical ram thereby increasing the amount of memory each resources can used and also by fine tuning programs and virtual memory will result in better efficiency of the overall performance of the computer.