Listen to what Yokohama is saying. It is your best bet. If you are a little PC
knowledgeable I would suggest you:
-Delete as much unneeded from your disk, diskspace is important for performance
-Once you clean up, run defragmenter, even better, use diskeeper from executive software, it will speed up your boot, it has a boot function and will do much better that the build-in defrag.
-Make sure you set your cache to be controlled by you (not the system) and set it to twice memory size (for you 512 Mb) work to get it in one contiguous space.
-Run spyboot to remove any ad-ware and spy-ware, also use one of the tools to prevent unwanted stuff to start at boot. Not all is needed, some can be run when needed.
-If you can afford, get more ram, maybe you can get it cheap on the internet.