If you do anything with computers then you probably get this all the time.  It seems like once people get a computer, even though they come to depend on it, they exert very little effort to maintain it, leaving it open to viruses and spyware.  I always get asked to take a look at these computers and a significant part of the time this requires the use of the original Windows CD which the owner never seems to be able to find.  After being stuck at a computer this past Monday with no way to get it booted and a corrupt dll that needed to be replaced, I decided that I should start putting together a "tool kit" that I could have ready to be more prepared for these free service calls. 
    The first item I have added to my toolkit is BartPE.  Run, BartPE, point it at a set of Windows install files and it creates and burns an ISO to either CD/DVD.  This CD is then a bootable version of Windows that can be placed into any computer and booted to.  You then have access to a functional OS with explorer access to the problem computer's original Windows partition.  BartPE allows you to include many other programs on your bootable OS such as a virus scanner and Adware which can come it handy when someone hasn't been taking care of their computer.  I checked several reviews of BartPE and most people gave it five out of five stars.  I think BartPE is one of the best things I've come across lately and I plan to make it the first and permanent addition to my "toolbox".  Feel free to post a comment with anything else you think would make a good addition.