Page file usage depends on a lot of factors. If you use a lot of programs and don't have enough RAM for all of them at the same time, then parts of them (or all) may be swapped out to the page file.
If you have 4GB of RAM and let's say around 3GB of it is available due to video cards and other resource users blocking out parts of the 4GB, and you only use a few programs that don't take a whole lot of memory, theoretically you should never need a swap file. However, Windows Vista and 7 are notoriously aggressive in their swapping of programs to disk.
I used to have a setting that reduced the swappiness of Windows 7 (swappiness is a Linux term but it applies to Windows just as well, and since there is a registry setting to tame Windows swapping aggressiveness it seems Microsoft agrees grudgingly, albeit in a well hidden manner), but I cannot seem to find it anymore. Searching for it is how I ended up here. The best alternative to this setting that I have found is to make sure that you use a fixed size swap file (I go for 4096-4096 on 32-bit OSes, and 12GB on 64-bit ones as I have 6GB of system RAM) on a dedicated partition. Having a spare 7200rpm 200GB drive, I place all of my swap partitions (windows XP, 7, 7 64-bit and various Linuxes) on this drive. It seems to work for me, but YMMV.