I hope you told your client all these other damages and they specifically declined those repairs and opted for just the CDRom and OS reinstall.
Otherwise, you need to tell him you will look at the machine (for free) and *if* the new problems are related to the work you did, then you will repair it, for free. If, however, you get there and some other piece of hardware is failing or it is a problem you told them about and they refused the repair, then give them an estimate.
Ask you asked about legalities, legally you are only responsible for the work you did. Also, you should have a signed work order specifying that the work was done to their satisfaction and has been returned to them in working order. If not, then you need to get on it as that is your biggest help if it comes to legal problems as it specificaly says they agree that it was working when you gave it back to them.
Either way, unless this turns out to be related to the work you did, don't try to impress them. They are already unhappy with your work and they will have a negative view on your business. If it is related to the work you done, then you might be be able to turn this into a loyal customer and great free advertising by showing how you fully stand by your work. Even then, your abilities are still going to be questioned because you gave them back a computer that stopped working not long after you gave it back. Nevermind if it was or wasn't your fault, most clients will not understand this or honestly care. If you look at it from their end, you can understand if you apply it to something you don't really get or are good at ("All I know is after I took it there to have the brakes changed, it has been eating gas like crazy ever since").
Just a note on how you can turn it into a great customer, I replaced a power supply for a client of mine and it died 3 days out of the warranty I gave her for it (30 days). When I looked at the machine I know she didn't touch it (I put a ziptie on the case so they can't open it) and the PSU was completely dead with absolutely no signs of life. I replaced it for free as I know for a fact it was unrelated to her or anything she did, got the new PSU in the next day (yay newegg and living near their distribution center) and had her computer back to her within 24 hours in working condition. She alone has referred at least 10 clients to me, not bad for "losing" $45 (technically I broke even cause the labor to install the dead PSU paid for the new one).