You need to clarify what you've downloaded. For example, have you just downloaded skins without downloading the programs that make the skins work? If so, that's your problem. WincdowBlinds and a number of other apps do work on W98. But if you download a WindowBlind skin without the WindowBlind program, of course it won't work. Ditto for most of the other stuff, there are programs that make the skins/themes work so you need the programs first.
There are some programs here that won't work on W98 like bootskins and logonstudio, for example. For that you would need XP.
The only way to upgrade W98 to XP is to get XP and load it on your PC. (And if you have an old PC XP may or may not be suited for it.)
If you simply want to make the W98 desktop look like XP, then you need some programs and skins for that as noted above.