Best Utilities To Speed Up Slow Pc's

There can be many reasons for a "slow" computer. Your post is not specific.
Low-end CPU, insufficient memory(RAM), overfull or severely fragmented 
local( C ) Disk, virus, spyware, and unnessary background Services all
contribute to the problem. If the "slowness" is internet related then the
speed of your connection, your browser settings, and DNS server all can
play a role. Rather than expecting some magical all-in-one application
to do it for you take this opportunity to investigate each of the above
categories to determine what you need for the way in which you use your
computer. These forums are a good starting place.

Do you have a clean-up utility?
Anti-Virus?
Anti-Spyware?
Firewall?
Do you defragment?
Do you know anything about XP Services?
Do you know how many programs start with Windows?
Can you control them?

Answers to all these questions will give you the ability to configure Windows
and your machine to your liking.