Does the computer start up in Safe Mode OK? It may be something as simple as your monitor not being able to handle the resolution. What monitor are you using? When you get the black screen, are the indications that Windows is working fine and you just can't see what it's doing, for example if you stick a music CD in the CD drive does it play (can you hear it)? Autorun should be enabled by default, but if nothing happens for a minute or so then possibly Windows has popped up that "what do you want to do with this disk" list and hitting Return should activate the default action which will most likely be to play it.
If the black screen and the CD test indicate that Windows has died then the problem could be a hardware failure. Windows should work out of the box if your hardware is good. Since reinstalling the OS doesn't fix the problem the next thing you could try is to remove ALL devices from the motherboard except (a) one RAM chip (and if you can test with a new RAM chip that would be better), (b) the hard disk and (c) the graphics card. Similarly if you can test with a known good graphics card that would also be better.
If that works then you know the problem is with one of the other devices that were attached and you can reattach them one by one, rebooting each time, until the problem occurs, then you know what needs replacing. (Makes no difference that it all worked fine under Win98. WinXP is a substantially different OS, not a minor upgrade, for example it is the second generation of the merger between the old Windows 9x home OS and Windows NT professional OS; the first was Win2000).
Just one other thing I would mention, since this was a Win98 PC it's possible that it just doesn't have the power to be able to run WinXP, never mind to do other things you want to do with the computer. Consider - and I mean seriously consider - not upgrading from Win98 in the first place or if you feel you must replace Win98 then consider replacing it with a fairly lightweight Linux distro that will enable the computer to act as a server.
Have a look at the minimum spec for XP at http://www.buildyourown.org.uk/pc-in...ng/windows-xp/
and bear in mind that the minimum spec means that XP will only just be able to limp along with the CPU maxing out to 100% for ages for every slight operation; if you're not at least DOUBLE ALL the minimum specs then don't install XP, or if you must have XP, upgrade the hardware. PC hardware is very cheap these days and you can easily get an XP-capable PC for a couple of hundred quid.