Is there a running XP system on that HDD that you want to upgrade to Win 7 maybe? Or do you just want to transfer data like files (pictures, music, movies, documents..) but not programs and saved settings?
If it's the first, there are tools for "Migrating Windows XP to Windows 7" (google it out a little, also you'll need USB Flash disk).
If it's the second you should be ok (assuming the old HDD is working fine, no bad sectors or corrupted data).
Hope that helps.