As far as win98 goes, I would say avast is your best bet as it is a very small list of AVs that support win9x or winME, it has nothing to do with ideal the limitation is win9x not the AV, there are many forum members still using win9x.
The point of going into safe mode is that some malware doesn't run and that is why avast couldn't deal with it in normal mode (file in use or protected, etc.) and why FWF suggested safe mode. Being in safe mode should give you a better shot at renaming the files detected in normal and safe mode than they would in normal mode.
These tools might also help:
- MoveOnBoot
http://www.download.com/EMCO-MoveOnBoot/3000-2094_4-10397293.html- Unlocker
http://ccollomb.free.fr/unlocker/ is also good as it also has a few additional features to not only delete the files but stop any process that is stopping you from deleting a file.
You would have to check if they work with win9x as there is less an less software that will work with old OSes, especially security applications.