Can I use an AVG cable to hook two compters to one monitor?

What you need is called a KVM switch (Keyboard/Video/Mouse).

http://www.google.com/products/catalog?q=kvm+switch&oe=utf-8&rls=org.mozilla:en-US:official&client=firefox-a&um=1&ie=UTF-8&cid=7593025254410011403&ei=QVxKTeyiMcGblgfzoIUv&sa=X&oi=product_catalog_result&ct=image&resnum=6&ved=0CH8Q8gIwBQ#

That link doesn't work for me. If it doesn't work for others, here's the KVM selection available on Amazon:

http://www.amazon.com/b?ie=UTF8&node=13983771

Probably a dynamic link, then.

I just googled "KVM" and posted the URL for one of the shopping results.

I hooked them up and my monitor has buttons which can used to select which machine I want to use. Unfortunately, all of a sudden the old computer doesn't react to the selection & no matter what sequence of buttons I push I only ever get the new computer.

This has the added difficulty that since I can't access the old computer I can't turn the thing off. Frankly, I'm nervous about simply pulling the plug, for fear of upsetting both computers in some horrible way and then I'd be really stuck.