Firstly, congratulations on spotting an absolute bargain. Quark Xpress 5 is the 2002 version of the program but it's still �880 of software for under a fiver!
Firstly, write down the number which is printed on the face of the disk. If your PC is not connected to the internet, go online. Insert the disk into your drive and click the 'Install' button. A new window should open, labelled 'Step-by-step: How to Install Quark Xpress'. There's a scroll bar at the right of that window. Scroll down to the end of the first section and click 'Register'. That will take you to the Quark website. On the website, click 'Create an account' and complete the required fields. Then click 'Create registration' and enter the serial number you copied down from the front of the disk.
Check your e-mail. There will be 2 messages from Quark. One of them contains your validation number.
Return to the 'Step-by-step' window and scroll down to the end of the second section. Click 'Install'. When prompted, enter the validation code. At the end of the installation process you'll be asked if you want to register your details for technical support. (It's optional).
Then scroll further down the 'Step-by-step' window, to the end of the 4th section. Click 'Install' to update the software to version 5.01. (The program will still work if you don't do that. I've just realised that I've not done it myself!).
Lastly, unless you've got experience of Quark Xpress, head to your local library to get a book explaining everything. (Quark Xpress is a superb program, which is used to produce many newspapers, magazines and other publications throughout the world, but you won't get the best from it without doing a bit of study).
Chris