I made my explore as 'underline icon titles only when I point at them' (in Tool > Folder Option > Click items as follows). I know somewhere in registry the 'hold to select time' can be changed, so it can be mouse over for 100ms and select files instead 500ms for the default setting. Does anyone remember that?
Why hack the registry when you can do the same thing (and much more) easily with TweakUI from Microsoft's PowerToys for XP?
If you really want to get your hands dirty, here's the relevant registry key and value:
You'll have to log-off (or at least restart Explorer) for a manual change to be effective though (no need for that with TweakUI).