Shouldn't be difficult
but a couple of questions.
As this is a self build, how did you install Vista Business initially?
Why go for Vista Home Premium when you have Vista Business?
As to installing, the current Vista Business will not let a lower version Vista install over it and if you tried, when you insert the Vista HP DVD into the drive and restart to boot from the disk, you will get a message that you can't install Vista HP on the same partition.
I am unsure if that DVD will allow you to delete the existing partition that Vista Business is installed on. If it does you can do that, then a quick format, then install Vista. But again, why downgrade to Vista HP? Why not use the Vista Business DVD to perform a repair install if the existing Vista Business is having problems?
If the Vista HP DVD will not allow you to delete the Vista B partition, then all you can do is wipe the disk clean with DBAN, then use the DVD to reformat the disk.
Why do you think you would need to flash the BIOS?
Vista, any version, is supplied on a DVD not a CD, so may be that is why you can't find it, if you are looking for a CD.