PF usage is "page file" usage. The page file is a variable amount of hard drive space that is dedicated to your total system memory. When your free RAM (memory) is low, the computer will start using this space to store RAM instructions. Because hard drive access is much slower than that of RAM, this can reduce performance. On the other hand, it also reduces the risk of your computer locking up due to low amounts of free RAM. It can also be referred to as "virtual memory."
You can change the allocated size of the page file by right-clicking My Computer, clicking "Properties," the "Advanced" tab, "Settings" under the Performance section, then the "Advanced" tab. You'll see it at the bottom.
Windows XP uses the page file differently than other operating systems, so it is not rare to see significant usage. However, because the page file is significantly slower than your RAM, it is best to keep it close to the start of your drive and make it a set size, not variable. Generally, a system total of 2GB should be good for recent applications. If you run many more programs at once, you will need more memory. For example, if you have 1GB of physical RAM, setting your page file to 1024MB will give a total of 2GB of available memory space. The pagefile does not take the place of physical RAM, though, as it will drain performance of your programs.
Increasing the size of the page file is not an alternative to adding more RAM.