I had the same problem. Blank page with MS IE 5.1, also with Netscape 4.72.
Upgraded MS 5.5 - still blank page.
My problem was the following: I use a 'firewall' Norton Internet Security. That causes the problem with Avis. I 'enabled' the Internet Security and Avis's reservation worked fine.
On-line discount for rates in Europe:
Exclusive Offer for reservations
made via the Avis website...
Save 27% off Avis' SuperValue rates to Europe
An advance online booking is required using Avis Worldwide Discount (AWD) number K427400
Rental must start on or before December 15, 2000
Reservation must be made online by July 31, 2000
Cars and rates subject to availability