Solved: computer hard drive will not reinstall

[TechQuest7]

Hello.....I took my hard drive out of my Dell Dimension computer to try and gain access to it in my good computer(Dell optiplex GX1).I tried to install it via an installation program downloaded for the purpose and it did not read it(wrong cable for the purpose),so I put it back. Now it will not connect to the motherboard at all. Someone said I probably messed up the MBR and to try to gain access via DOS to correct it......I just can't get into it!!! Why?
What started this was it was hanging at loading personal settings. Thanks for any help and or advice!!!