A repair install will not repair a broken registry. Just system files.
If system restore has been turned on you can perform a manual registry
repair by following Microsoft KB article 307545.
http://support.microsoft.com/default.aspx?scid=kb;en-us;307545&Product=winxp
This is a good option if you are making many changes all at once in the
registry and you do not know where the bad registry key is, but you do know
when your system last worked properly.
Another option is to start a repair installation and then at the 39 min
remaining time stamp ( at the beginning of the GUI mode part of setup - where
you have multiple colors on the screen ) select shift +F10. This will bring
up the cmd prompt where you can type in "regedit" ( without the quotation
marks) and fix the issue this way. ( assuming you are trying to make one
registry change and you know what it is). Once this is done let the repair
continue and once you are done you will need to get your windows updates
again. ( The down side of this is that a repair install wipes out all of your
windows updates as well as your system restore points and then the first
option is no longer available to you. In any event, you will need to be quite
familiar with working in either the registry or in a DOS ( recovery Console
) environment