I have successfully figured out how to share files between WinXP host and Win98 guest in Virtualbox 4.0.8.
In the Win98 VM settings in VirtualBox, set the networking to Host-only Adapter.
In WinXP host, click Start, Run, cmd, ipconfig
Note the IP address under the Virtual Box Host-only Network.
This is the IP you will use if you want to connect FROM guest TO host.
In the Win98 guest, click Start, Run, command, ipconfig
Note the IP address. This is the IP you will use to connect FROM host TO guest.
In Win98 guest, share desired folder. If 'Sharing...' is not visible as an option when you right-click on a folder you want to share, then you'll need to do the following in Control Panel. Start, Control Panel, Network, Add, select Service and click Add, select 'File and printer sharing for Microsoft Networks' and click OK. I don't remember if you'll need to reboot or not, but it wouldn't surprise me if you did.
On WinXP host side, Start, Run, \\IPofguest\
to see shares on guest.
I was unable to access WinXP host shares from Win98 guest. I did try briefly turning off my WinXP firewall, but it had no effect.
It may be because my WinXP host cptr is on a domain.
I did not edit the Win98 guest network group from its default.
NOTE: The above instructions will allow you to share files between the listed host and guest, BUT the guest vm will NOT be able to access the Internet. If you need to access the internet from the guest, temporarily change the guest vm networking settings from 'Host-only Adapter' to NAT. Naturally, this will break file sharing between host and guest until you return to 'Host-only Adapter'. Also, its possible that every time you switch, your guest vm will get a different IP address, so you may need to do ipconfig again to see your new IP.
(someone may want to add this to the end of the tutorial in question)