Hi,
I also use Win98SE, but stick to Adobe 5.
Win98SE is very "funny" when it comes to Adobe. Even the website for Adobe Reader, Download Manager, etc, says certain versions are not for win98/98se.
I would follow the advice given by another and remove it from Add/Remove Programs. You might also have to do a little manual removing as well.
But, if you wish to try something first, here is a link explaining how to, possibly, fix the problem: (deals with your error message)
http://www.adobe.com/support/techdocs/324057.html
Best of luck.