Hi there, & Welcome to the forum!
On Windows XP, Outlook Express stores all your Contacts in a single Windows Address Book (WAB) file of type *.wab. The location of this file depends on which version of Windows you are running, whether Windows User Profiles are being used, as well as the upgrade path you have taken.
It not very useable just to know where it is located, but you can search for *.wab if you want.
In Windows 7 you have to have an email client into which you can import your Contacts, such as Windows Live Mail, or even Outlook. Then you can use the utility in Win-7 called Windows Easy Transfer, if the HDD is connected to another computer, or import them from the WAB if the HDD is attached as a secondard drive to your new machine.