You don;tr say what Operating System you are using but I'll guess Windows 98 or Windows ME. The reason that your machine slowed down is because the hard disk got fragmented. Try starting the machine and pressing F8. Then select SAFE MODE. When you are there select: Programs, accessories, system tools, defragment hard disk. It will take a long time so you may want to do it over night. Once it is done try booting again. The run disk Disk Cleanup from Accessories again and then do another defrag. If this doesn't work or you need more, do a repost with more information like what does it do instead of boot?
Terry, August 2006