need help uncompressing hard drive

somebody has asked me to fix their computer it has a compressed hard drive and the drive has been formatted but of course I can not load windows back on until the compression has been removed I have done this before a long time ago but I can not remember how it was done if somebody knows could they please help I am almost ready to give the pc back at him and tell him I can't do it. But I am sure I can fix it if I could just remember how. I need to be able to uncompress it in dos.
Any help would be most appreciated thank you.

 

I think it sets up an additional partition which needs to be removed...

If you run FDISK from DOS and delete ALL partitions, and then recreate a Primary partition, make it active, then format it -- you should be able to reinstall windows from there.