Sorry, you cannot remove this restriction. If you have more than
10 users who need concurrent access to a shared resource then
you have to move to a server environment, which is highly
recommended anyway in an office of 20 users. Have a look at
Small Business Server - it's competitively priced and probably
ideal for your environment. It will pay for itself in no time at all
for the following reasons:
- Centralised file storage.
- Centralised file backup.
- Centralised account administration.
- Centralised administration through logon scripting.

Windows 98 for the "server."

    Simply disable File & Print Sharing, and de-select the exception for
File & Print Sharing of the Windows Firewall.  All remote users
disabled, as requested.

    Don't see what this has to do with your question above.  Be that as it
may, if your client needs a host machine (server) that supports more
than 10 simultaneous connections, he'll have to replace the workstation
operating system (WinXP) with a server operating system.  Period.  If he
can't afford a Microsoft Server OS, there's always Linux.  It's free.
(Don't know if it can be made to support QuickBooks, though.  I rather
doubt it.)