Today (18th-August-2007), I saw cool tip on CNET TV.
Acrobat Reader is really great free program, but with every new release from Adobe it becomes more big (disk size), slow and really hungry for system resources. The main reason for that is adding new features. The problem is that most of user never uses them. As result many programmers start writing on PDF viewers. Some of them are really great, but the main problem with all of them is that they need time to be updated after each new release from Adobe. I found out that many time I can't open some PDFs with 3-td party viewers.
Thank to free program called 'Adobe Acrobat Reader Speedup' from Joseph Cox you can solve the problem. The program disables unused plug-ins, but at the same time allows you quickly to re-enable them when you need to use them. (Like filling PDF form, or reading the eBook, or etc.)