If you're simply trying to access all your data from a single PC, there's a simple way to do it. Just map a drive letter for each PC you want to see (assuming they're all on your network).
Each hard drive or folder will show up as a drive letter. This will go smoother if each PC has a Windows Login ID and Password. If you have no password, add one; otherwise you have to tinker with Registry settings. Your password can be a single character if you wish. You could also create a single new ID on each PC just for this purpose.
Launch Windows Explorer on the 'main' PC, pull-down the Tools menu (click the ALT key if you don't see this menu), and click Map Network Drive.
Browse to the PC you want to access, login using the remote PC's ID & PW, check box to make it permanent, choose a drive letter (T: for a Toshiba, I: for an IBM, or whatever makes the most sense).
You can map to the root of the remote drive, map to a second drive, or burrow down a bit to get to the My Documents folder, or whatever you prefer. You can map multiple letters to different spots on the same drive.
All PC's must be turned on for them to be seen, but they don't need to be logged in. Assuming the remote ID is an Admin, you'll have the authority to edit, delete, move or copy files (even to another PC), etc. You could open a Word doc from your W98 box on your 8.1 installation of Word, then save it back there when you're done.
This only works for data -- you cannot run an XP app (e.g., Word) from the 8.1 box. You could, however, copy photos and docs (or anything else) to back them up. If you wish, you can create a simple batch file that would copy entire folders on demand. Google 'Robocopy' (free) or write your copy or xcopy commands as each line in Notepad, save the file, change the extension from .txt to .bat and create a shortcut to it.
However, you can also install TeamViewer (free) on each PC and set them up to auto-accept a remote session from your main PC. You'll see each PC's desktop, control its mouse and keyboard as if you were there. You could even remove their monitors, mice and keyboards and just run them all remotely. This is valuable if, for example, you have an app that only works on W98, but you want to use it at your 8.1 PC. The work would be done on the W98 box, but you'd be sitting at the 8.1 box.