what do i do when i have and original pc e.g DELL or HP with windows vista OS installed, normally these pc come with a copy of the operating system on dvd. If i crash or mess up the pc badly i can normally do a clean install and all my info logo will be returned, but if the hdd dies an i replace it, how do i go about this problem beccause my os is not goin to be read as a bootleg if may.

in cases like this, what is the best thing to do ? or do i have to pay for a license all over again ?

No, you don’t need to buy it again.

Some times you don’t get a DVD with the system; in that case you burn your backup DVD(s) with the software that is included. If you hard disk dies just ask for a replacement and take your PC to a service centre, if it’s out of warranty then replace the hard disk your self and install the backup image or the DVD that came with the system...

Or am I missing the point here?

Moving this topic to the Vista forum.

No you didnt miss the point, you cleared up some of what i wanted to know . but in this case i was refering to a client's pc , his hdd went bad, of course he didnt create a back up dvd of any sort. when the hdd was replaced and the OS was installed clean the serial came up as invalid.
it wasnt me that was working on the pc though, i guy told me of it, so i like to have a heads up on these matters before i face them

You could call the service desk from DELL or HP, and say that the drive has crashed or something?