1 As you have removed the original hard drive, would it be fair to say that you don't need detailed instructions on how to fit the replacement hard drive and you are familiar with the personal safety and ESD ( anti static) precautions necessary?
2 If you are having difficulty with any of that then post back on the area that is causing concern and we will do our best to help.
3 When it comes to installing your operating system of choice, Linux, Windows, or whatever you will first need to obtain the installation DVD.
4 Linux can be downloaded for free, and then burnt to a DVD or CD ( burn the .iso file as an image)
5 In the case of distros of smaller size.... for example Linux Puppy, is only a tad over 100 Mb so will fit very comfortably on either a CD or a DVD whereas for example Ubuntu will definitely need a DVD as its file size is far too large for a CD.
6 To use these CD/DVD's set the BIOS to boot from optical drive ( CD/DVD) as boot priority one, with hard drive 0 as priority 2 , save the changes and exit with the CD/DVD in the tray, your computer will then restart and boot into Linux ( if you have done everything correctly!)... if you want to try out rather install then either will be happy to run purely in RAM without writing to your hard drive.
7 If you want to install Ubuntu (Rather than trying it out using RAM only) then follow onscreen directions.
8 With Puppy if you want to install then run it first in Ram as normal and then select install from the menu.
9 Both distros will deal with partitioning and formatting the drive for you... follow onscreen directions.
10 If for example you want to install Windows then decide on the version you prefer..... Vista runs out of security support from Microsoft in about a year so would only be a very short term solution, it may be ok if you have a spare fully licenced retail copy available, but if you have to buy it, then it might not prove to be a good deal....
11 Whatever version of W)indows you select (Win 7, Win 10 etc,) do make sure that your hardware is compatible before you spend your money. you don't want to buy an operating system that does not work properly on your older hardware. ( 32 bit would probably be advisable rather than 64 bit) you will need to do your research on this.
12 You could install Win XP to your new drive (you will need to obtain the installation disc) but it's difficult to use a computer running Win XP safely these days unless it is completely isolated from other computers, data sources and of course the Internet , without taking extra ordinary precautions. Installing Win XP is therefore generally not now recommended.
Post back if you need more detailed information on any of this