window vista --high cpu usage

just recently, my ibm lenovo 3000 became slugginsh.
Resource Monitor show explorer is consuming a lot of CPU...nothing
ran anti virus and no virus detected.
ran in safe mode, and system runs normally.
ran in diagnostic mode which has fewer servies running than the safe
mode and the cpu crawls. so it seems that it's not the running
services that are causing the cpu to slow down.
i tried to logged in as a different user, which i created fresh and
it's also slow.
In normal mode, even if all services and startup are disabled, system
is still slow.

i am thinking of purchasing Xp and install over the current Vista but
i am not even sure if XP installer will let me down rev from Vista.

does anyone have suggestions.

my systems:
intel 1G T2060 @1.6G
1 G memory
SATA 120G hard drive

thank you in advance.