Certificates problem - FreeRADIUS 1.1.6, Chillispot 1.0

[@eon]

I have setup a hotspot for my hotel using Freeradius and chillispot. It
works for all clients normally, except for two cases I've seen (so far). Two
different users with laptops running Windows XP could connect to the access
point, could ping the server's ip address, but when they tried to open a web
page, their browser (internet explorer and firefox alike) did not prompt to
accept the server's certificate and of course could not see any web page.

I've tried looking around at the windows' settings for certificates and
security to no avail.I know that the 2nd and more recent case, the cliend
had windows xp SP1 on his laptop. Could that be the case?

Thanks in advance for any answers to this.
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