OK, I probably can't get you past the check sum issue with the BIOS. And the answer to your problem may have something to do with the age of your computer. But I see you've been waiting a day so here are a few thoughts that might help.
The first thing I would do is boot up the machine and see if I can get into BIOS. If you can, tell the BIOS to redetect the primary drive - assuming you've replaced the old drive and not simply added a new one. Then save and your machine will restart. That's the quick and easy solution. And who knows? Will probably clear up the checksum problem. If this doesn't work...
Go to your manufacturer's site (like Dell, Gateway, eMachine (also Gateway), HP...). In your case Compaq? The site should have your original set up, but you have to know your model number or serial number or order number, etc. If your machine is old you may have to look in the Archived support area. But from your note, you may be past that. You should be able to get all the specs, including the BIOS manufacturer here. The user's manual may have a way to clear/reset BIOS via a jumper setting. (I THINK that's right.)
When you flash the BIOS, you have to be very careful and follow instructions exactly. Depending on which BIOS you have (Award, Phoenix, AMI, etc.) you may be able to get help on their site if not Compaq's.
Now, as to the HD, you need to have the correct cable, it needs to be attached properly, and your machine needs to be able to handle it. You may not have this problem but here's a very nice (old) site that explains:
http://www.mikeshardware.com/howtos/howto_connect_ide_hd.html
Also, with cable select (CS) the problem of setting up the BIOS should be gone. But if you have a very old machine, you may need to go into the BIOS and specify your new drive. Also, I believe some older machines simply can not handle large drives. I don't know if simply upgrading the BIOS can overcome this. It has something to do with HD sectors and that's all I know.
(I think there's also something weird about Compaqs?)
I think that should cover it. But to get better help, post your system specs. At least the type of Compaq. BIOS name and version, number of drives: DVD, CD, HD.
Good luck.