Did you reformat the harddrive and reinstall windoze?
There's alot of stuf windoze setsup for in the background that will only work on the origional computer, so you need to format and reinstall the OS while it's in it's new home. Often a drive won't work in a different computer just because windoze can't recognize the hardware in it (with your other computer's drivers installed).
Also, while it's booting, does it specifically mention the type/size of it's new harddrive?
If not, it may be too large depending on how old the compaq is. In this case, you would have to partition the drive using a windoze boot disk, into smaller chunks that the old computer can recognize and work with.
Let us know how it turns out for ya, bud.