To be honest you should be fine unless you use the same serial key on the same computer a few times, as it may say this key has already been used and if you make a few hardware changes like i did you get something like...
"this computer has gone under significent hardware changes, please contact microsoft or your system admin"
but if you call Microsoft and say your old PC died, majour hardware failure, ive upgraded a few parts etc etc they should give you a code which you could use on your new PC
TBH you should be fine as enough people do it and Microsoft know they do!