Best bet, burn as many utilties as you can find for every operating system your
support to CD or DVD. Better yet, copy them to a USB thumb drive. What you'll want is a
collection of programs that are specific to some kind of probable failure. You'll want to
update them from time to time. This necessarily entails searching for the utilities
you think might be useful, trying them out and then keeping the ones that seem really
useful. Once you've accumulated a couple Gigabytes of these programs, burn them to
CD/DVD, label them with a meaningful description and stash them someplace safe.
You don't really have to make the CD/DVD bootable but it might be handy if a computer
can't boot. You can also have a "Live CD" like Knoppix to boot from but if you're
wanting to repair a Windows installation it might be better to create a bootable
Windows CD. Keep in mind that Windows has different file systems for different versions
so you should have one bootable CD/DVD for each file system type. I've used the
Ultimate Boot CD (get it here) and it's
got most of the utilities you probably need. If you need more, keep browsing.
Create a directory on a partition separate from the Windows partition as a repository
for all these programs (for more on disk partitions, see this). Make that directory the default location
for your browser downloads. Select that directory as the source for the CD/DVD copy
and burn it to CD/DVD.
Since some files don't require modifying the Windows registry,
go ahead and install them and then copy the entire installation to the CD/DVD. You
should backup the Windows registry soon after the initial installation, before it's had
time to get corrupted and copy the backup to the CD/DVD as well. What you want is as
much useful stuff as you can fit on a CD/DVD. With an 8Gb DVD you should be able to
stash every utility and diagnostic program know to man. Better get started.