Some times Registry is disabled in Xp due to virus or whatever... Here is a way u can re enable it
You must be Admin not a limited user on ur machine for this.
Goto run type gpedit.msc
Now on left pane select User configuration
then select Administrative templates
then select system
now look on right pane
Double click on Prevent access to Registry editing tools option
select option Disabled
thats all Registry will be enabled.