Once a month...wow...why do you need to do that? Is your system crashing or something? That would be the only reason I'd do that, or if my system is getting noticably sluggish, and it needs to be cleaned out.
I'm curious, are you like tight on space? Everyone wants a fast system, but there's a point that the amount of gains you can get is outdone by how much work you have to put into it.
I'd recommend Windows XP, because it's very stable, if you set everything up properly from the get-go. It handles memory very well, ends "DLL Hell", and gives decent compatibility (read: basic) right after installation. Once you set it up perfectly, you should image this to another partition, hard drive, or burn to CD. That way, you don't have to do this once-a-month Windows installation. It'll save you at least 10 minutes, depending on how much you add to your Windows installation.
If you stick with Windows 98, please post any details on your computer habits, what you install, what your computer's specifications are, and anything else you'd like to divulge.
<font color=orange>I feel like I'm looking into the past, about one year ago...</font color=orange>
<font color=red><A HREF="http://www.geocities.com/btvillarin" target="_new">My Windows XP-based Website</A></font color=red>