If you have typically had errors in the past, and now have a relatively stable system, you can easily 'reset' the scores - although this is not technically what you *should* do.
However,r for those interested, details below.
Some background - I have been running Vista x64 for a little over 2 months - and have been trying out programs left and right, as well as OCing my system, testing stability, etc. This has led to overall low reliability scores for me, so now that I Have reinstalled and fixed the 2 remaining issues I had with my OC and stability, I wanted a quick and easy way to 'start from scratch' without having to reinstall my OS again.
I search the Internet and found a post by a MVP that detailed how to go about resetting the Reliability index. I'll quote it here for ease.

Quote: Originally Posted by
http://www.vistaheads.com/forums/microsoft-public-windows-vista-general/111069-reliability-performance-monitor.html#post406720 This can be accomplished, but it is not really recommended. One of the
biggest problems when troubleshooting system problems is determining what
happened and what caused it to happen. Reliability Monitor gives you a day
to day analysis of application and driver installations, records failures in
applications, hardware, and the operating system. Reliability Monitor will
be valuable when troubleshooting problems because it allows you to quickly
see changes even if the user is not aware of them.
The Reliability Analysis Component (RAC) calculates a stability index over a
period of time and tracks changes that are likely to affect stability. It
displays this information in the Stability Chart. This chart contains
information that goes back to the time when Vista was last installed.
If you still want to reset this important information, open Windows Explorer
and navigate to the C:\ProgramData\Microsoft\RAC
Folder. There are 2 sub-folders named Published Data and State Data. Copy
these 2 sub-folders somewhere safe and then delete the contents of the 2
folders.
The Reliability Monitor will show as empty until it has checked the system
which happens every 24 hours.
If you decide that you want the information back, just copy the 2 folders
you saved back to their original locations.
Link to original post:
Reliability & Performance Monitor - Microsoft Windows Vista Community Forums - Vistaheads
Brink - tutorial?