My external hard drive is not recognised

Before I start I just want to say I have read all the related issues and although people have had similar problems to me I have tried all the solutions that have worked for them but nothing has worked for me. So now that's out the way...

I have a Packard Bell Store & Save 3500 external hard drive that I've been using with my desktop computer for about 3 years now. It's always worked fine before but recently I plugged it into my new laptop which runs on Windows 7 to transfer some files, and now when I plug it in to my desktop PC which runs Windows XP it doesn't show up in My Computer. It shows up in Device Manager but not Disk Management on my desktop computer, and shows up in both on my laptop but in Disk Management it has a red X on the image and says it has not been initialised. When I try to initialise it, it then comes up with an error message saying 'device not ready'. I really really don't want to reformat it because I have files that I want to keep on there and other people say this has not solved their problem. I've tried everything else I can think of and would really appreciate some help on this issue! Thanks in advance.

PS. Oh, forgot to say, I don't need it to work on my laptop any more I just want it back on my desktop PC! Thanks, Helen.