Due to important data on hdd ,
1) buy a hdd of same or greater size
2) get a good clonning software - there are 2 types, one you need will make a exact copy (bite by bite) of hdd, not one that copys usable files only. BESURE you do NOT clone the new hdd to old, have some one that is more knowlegable do it (if needed).
3) after you make the clone put the orignal hdd aside for safe keeping.
Question: you said that the hdd was in a MAC OS X comp before, was you only able to read only or could you write to the hdd ? If read only the hdd was likely NTFS, it you could write then hdd had to be FAT32.
With the new clonned hdd would run TESTDISK , I have only used the dos version not XP version. The program will verify the Partition table and Volume Boot Record, where your problem is likely at.
http://www.cgsecurity.org/index.html?testdisk.html