This is odd, are you sure you've really reset the router to defaults (see also post Antiloop). AFAIK this should do the trick.
The only minor problem now outstanding is that I have to leave the Logon "User Name" blank and just enter the password, because, if I enter the "User Name" as "Admin" and then the password, I still can't log-on!
Yes you do if you want to do this:
probably don't need any of the advanced features that the tweaked software may provide.
The original Asus FW's only support FTP but you likely would appreciate Samba, which supports sharing your HD over your network. Samba is only supported by Oleg's custom FW's, see http://oleg.wl500g.info/
connect an external hard-disk in a USB-IDE or USB-SATA enclosure to provide Network Attached Storage
You could start with this thread http://wl500g.info/showthread.php?t=1984 but other threads about Samba can be useful as well, just search forum.
However, be aware of the limitations. Some people, like myself, have reported problems with copying larger files. Also relevant to be aware of is that file transfer speeds on the whole system are not that high, see for instance http://wl500g.info/faq.php?faq=asus_...wdshowfastisit, but this is not specific for Samba AFAIK.
In general, just check the tutorials section of this forum and the FAQ. You'll learn a lot of the capabilities (and limitations) of your box
Stuff I mess around with: Asus WL500gx (18.104.22.168-7g + new bootl. - on USB: HD + HP DeskJet 843C) - Notebook (Medion - Intel Centrino Pentium M-735 1.7GHz, 1280MB, 100GB - Intel PRO/Wireless 2200BG + WPA-PSK, XP-home) - Desktop (Medion PIII-1GHz, XP-prof, Lan) - ADSL2+ (Livebox in bridge mode)