How to reset checksum without booting laptop

Well, this is an older article last updated in 2008 and assumes that you are running Windows XP, but the information it contains may still be applicable.

http://support.microsoft.com/defau [...] -us;326687

Basically to sum it up, it could be a bad memory module causing the problem or you could have a damaged file in your windows folder.

Seeing as it's a laptop, if it's the memory, you can try removing the memory
(you should use an anti-static wrist strap when doing this) and make sure you are grounded so as to make sure you don't kill the memory with static electricity.

Look to see which slot is marked DIMM0 and which is DIMM1 - you'll want to insert one stick of memory in the DIMM0 slot then try booting your laptop. If this does not work, then shut down and remove the module and try the other module in the same slot.

Hopefully it'll be nothing more than one of the memory modules gone bad.

If you still get the same problem after checking both memory modules, then insert both memory modules in their sockets and close the cover then use your Windows installation disk or if possible, your recovery disk, then boot from the disk optical drive and do a repair of your existing windows.

Doing a google search for stop: c0000221 {bad image checksum} the image rpcrt4.dll basically pulls up much more of the same

One other thing to consider if this is an older laptop is the hard drive may be failing.

do you have your operating system disk?
BTW: keep in mind doing so will erase everything on the computer.