Well..I would strictly recommend that first "suggest" the client to upgrade the OS to a later version[May be XP].I understand that the client may have thier own story for sticking to win98..but try to convince them that the win98 is already an outdated OS.
http://www.microsoft.com/windows/support/endofsupport.mspx
http://www.microsoft.com/windows/lifecycle/default.mspx
It's recommend not just because MS has closed the support for win98 3 years back but the fact is there are many hacks available for win98 which can be used even by a "script kiddie" to "break" into the system and the OS vendor would not take any responsibilities coz' they've already announced the end of support long back.
Take my words for this, I've seen cases where because of "one" missing updated and outdated software the entire network is compromised ! An AV software is at is best when an endpoint meets the "minimum baseline of security".
SAV 10x 9X are personally my favorite choice and Win98 is one of the most "appreciated" OS...but the "security trend" has been changed my friend..and we gotta be "up to date" !