rundll32.exe...help

Whatever this program is rundll32.exe it is screwing my computer up. Several weeks ago I had a lot of programs on my computer just freeze while I was using them so I would try to log off or turn off my computer and get back on again. Right before it logged off, or turned off a message would come up saying that rundll32.exe was not responding. so I logged back on and went to search for it. I found some files named that so I went to delete them. I thouht I fixed the problem because my programs stoped freezeing but.....now I cant unistall anything on my computer and the control panal isnt working, I get a message that "windows cannot find 'rundll32.exe' etc. Under help and support it says insert  the windows xp disc but I dont have it, so i dont know what to do. Any help would greatly be appreciated.