Software for a low end system. Any suggestions?

Hello
I'd like a few opinions if thats possible

I'm currently rebuilding/modding some old laptops for fun, and am now going through the process of loading them out with some standard software (firewalls, virus killers etc)

The 2 laptops with the lowest specs are P1 233MMX, 80Mb RAM, 4Gb HD (formated NTFS). If your really interested you can find more info on them here: http://www.orphanlaptops.com/mp975.htm

I did have Win 98SE on them, but have switched to Win 2000 Pro.
Anyway....the point is I'm trying to find the most efficient (especially when it comes to RAM usage, then CPU usage) software.

I'm also trying to find a balance of usability and stability, which is why I switched to Win2k...although if anyone thinks I should go back to Win98
I'm not adverse to putting linux on there.....but I'd like any novice user to be able to pick it up (hence going for windows, being the most widely known OS)

Appart from the OS...
I've installed Fast Defrag 2 Free to help keep ontop of its low memory resources....it seems to help quite well. But am I misguided in thinking that I need this? Is there anything else that is better at freeing up ram automatically in teh background? (I'd love something that just idled and free'd up a few bytes ever second or so)

I've tried Zone Alarm free as the firewall.....unfortunatly its too slow!
AVG Free didn't fare much better....although its bareable.

I've installed the latest version of Opera as the web browser, and memory/cpu wise I'm quite happy with it. (Although I may need to tweak down some of its graphics settings)

I was going to plump for Open Office as the main office type software....but havn't got that far yet.

I've also found a few programs that defrag your HD in the background. Ashampoo Magic Defrag does it when your systems idle (although I've yet to test it on this system, it seems ok on higher end systems...but does sometimes not stop when you want todo some work! slowing things down)....the other is HS Defrag Saver...which goes to work when the screen saver comes up.
The idea behind using one of these is to keep things running as fast as they can...given the average user doesn't tend to defrag there system...ever...
Again...something that just ticked over in the background...and took maybe a week to defrag the HD would be great! (if its running in the background constantly....would it matter that it took a week...)

I'm going to keep playing about with different software, but thought other peoples input might help speed it along

On a side note....if I can find the most efficient software about....I think I may use it on my high end systems just so they fly along even faster