I guess the pc came with Windows XP, pre-installed.
You can use the number on the desktop case ad the back or in the CD/DVD hatch in front (when have any hatch). You could (don't know now) usually put in that number and you got the drivers download shown ad there website.
Set the website to the language you prefer, than choos Support & Drivers.
There you have two options:
Copy or type the pc model (or that number) into the search bar and press Enter.
Here you will find all the drivers you need and much more.
You might like to try to copy or type:
HP Compac 7100(??? or not) desktop pc Windows XP
It give me a different out come.
Or automated product search ans solutions.
Here the firm will check the system you have and will give you options to choos from for installations.
Or you have to download a little driver inspector that will find the stuff you need. I'm not shore about Windows XP, you have to find out yourself and let us know please.
These days, a lot of companies use such little program.
Well, good luck with it and have lots of fun doing it!
Hopefully I let you known enough and that you will find the answers you seek!
When you have an audio card and VGA card installed, you have to find out wich one you have and when not to old, you can download them ad there firm website or ad a drivers collector website.
Just search the net for it like:
Realtec audio (?) card driver for Windows XP (x32 or x64)
If you don't know wich ones you have, download
SiSoftware (the free edition)
Lavalys (the free edition)
When lucky, they have both links to the product website and/or driver update website.