I have 2 PC's with two seperate hard drives both with a lot of data on them. Can I set one as a slave and install it in the first machine.
So when I boot the machine the second hard drive will show up as an extra drive and I can retrieve docs from it and store them back to there??? Basically what I want to do is use the one PC but keep the data on the 2 hard drives seperate.
Or will I have some issue with programs working using the second drive?

you can swap hdds no problem, as long as the physical connections are compatible (pata/sata), and the hdd jumpers are configured properly (if its pata, on each ide channel, one should be master, and the other should be slave, cable select also works too usually, whichcase you should set both hdds jumpers to cable select), if theyre sata hdds, no jumpers are involved, since every sata hdd is a master hdd on its own channel, which eliminates possible jumper complications

as far as programs working, if one program was installed on one computer, it might not work on another computer without a reinstall/repair of the program, due to possible missing files/missing registry entries that might be needed for a program to run properly.

beyond any of that though, actual data transferring between hdds shouldnt be a problem, because once its formatted as fat32/ntfs, it should be recognized in any other windows installation just fine

I'm assuming that both OSs are XP? XP can see fat32 but win 98 can't see NTFS. Yes the drive can be added as slave. Programs on the slave won't run properly. They will need to be reinstalled preferably on the master "c" drive, do to registry etc., as was said.

i knew windows 98 couldnt be installed on ntfs, but i thought it was still able to read from it, hm, maybe not

edit: i guess its not... though i googled and found this: http://www.mount-everything.com/ntfsw/

it allows non nfts supported windows os' to access ntfs formatted partitions as if they were still fat

Thanks for that.
Both hard drive were running windows XP medic centre edition and the programs are replicated on both machines.
They are SATA hard drives so do I need to change jumper settings on one of them to make it a slave??

If they are both SATA just plug and play.

Fantastic - some it will auto detect the second one as a slave and still boot from the first one?

There's an OS loaded on both so I thought it might get confused and not know where to boot from??

It will still boot from the first one because that is controlled in BIOS. Turn off auto update and remove windows defender if it is installed. The reason for that is that MS may disable booting to the second drive because they will think you stole it and invalidate it. If you aren't ever going to boot to the slave then don't worry about it.

Paragon makes nice software. I have used their Disk Manager partition software and it worked fast and flawless.

These days not too many people are still using 98. What I did years ago was set the XP drive with a second partition that was fat32 and put the files that I wanted to access with 98 on that partition. I liked 98 because of the Kodak Imaging program that they didn't include in XP. Alas, 98 bit the dust several years ago.