I don't think "you" buggered your PC, but your brother certainly did.
No, although removing an EIDE drive while the PC is running, is not recommened, it shouldn't actually make your PC run slow or slower after this is done. Removing an EIDE drive while windows is running might have caused some type of error in windows. EIDE drives are not hotswappable, nor, can they be, "removeable drives".
I would recommend that you open up the PC. Look for a small silver battery. It looks similar to a BIG watch battery. Remove that battery for 10 minutes or so. Then pop it back in. This will clear the CMOS and return all of the BIOS settings back to default. Which could be your problem. Your BIOS could be in flux stumped still thinking that the EIDE drive is still there... Yet it's not.
MAKE SURE YOUR PC IS OFF AND UNPLUGGED FROM A POWER SOURCE!!!! Then reboot your PC and let it P.O.S.T. Allow it to boot to windows. Let me know what happens...