System 99% cpu usage

First, you must make sure that you have at least 512 MB or more RAM memory installed, XP will not run with less, and it will freeze up the processor, as you said at 99%
However, It is possible that the processor or motherboard, memory is defective.
Start the computer, allow the entire system to boot up without clicking anything, (until the blinking light stops blinking)
NOW does the processor show 99%?

The thing is that manufacturers such as HP, Gateway, Dell often sell computers with the absolute minimum amount of memory- and it's not enough to really use the computer.
The computer DOES turn on BUT...
The minute you start trying to use it, the memory overflows, the processor goes 100%, and the whole system locks up!
or, is a slow as a snail on vacation...
You need to figure out what mother board you have (open up and look at the inside board model number)
then you can pick out the right type of memory. Don't guess.
You really should have at least 512 MB, and I'm thinking HP installed 256 MB, as usual.
Really 1000 MB would be better
256 would explain WHY the processor is locked at 99%!
AND if it's an HP Pavilion, many of those do not have processor cooling fans, and the processor gets really damn hot.
I mean hot as Hades, hot enough to crash the whole thing.
It's not a bad idea to retrofit a heatsink and cooling fan, it makes a HUGE improvement for about $30 or less. It's almost like a new computer with some memory and a fan added. I have worked on those %##(*!! Pavilions before.

So lets find out your Computer Model Number and Type,
and if you can, look inside and let's find out the motherboard model number

Then we can recommend some memory sticks, and a vendor and price.

And if you don't know how to open this thing up, find a friend with some computer experience to help you.