theoretically, you can do as you wish. see support.microsoft.com for 'reinstall windows 98'. for many good articles including how to refresh windows without clobbering all your installed programs etc. but my 2 cents is you are asking wrong question. i have never had to reinstall windows to fix it. now that it is unsupported os maybe not true but betcha if you post errors we can fix.
uninstall the mp3 software if you have not already done so. ask the techs for manual removal instructions if you can stomach calling them again. with the advice they've give you so far it is unlikely they have such instructions but maybe worth asking.
start out by seeing if system is stable in safe mode. if not, think 'hardware problem??'
perform all windows updates
perform spyware removal with tool of choice. mine is ad-aware personal se from www.lavasoftusa.com
if you do not have anti-virus protection, download and run stinger.exe from www.mcafee.com then get yourself some antivirus and confirm system is clean
if still no joy after that, read up on how to restore win98 registry files at support.microsoft.com. perhaps you will have a fairly recent registry backup that can bail you out of this. read many other troubleshooting articles there
