Help!a bout PM1.25

when i send a pm to a user
the page displayed:

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Warning: Server Error in c:\inetpub\vbulletin\private.php on line 521
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but pm can be sent successfully.
why?
how can i fix this problem

btw:my system is Win2000 server+php4

Line 521:
mail($touserinfo[email],$emailsubject,$emailmsg,"From: \"$bbtitle Post Manager\" <$webmasteremail>");
You're mail functions aren't properly defined/configured in php.ini. You'll get an error in any program that tries to mail.

in my php.ini

[mail function]
SMTP = localhost ;for win32 only
sendmail_from = me@localhost.com ;for win32 only
;sendmail_path = ;for unix only, may supply arguments as well (default is 'sendmail -t -i')


but i have not install smtp on my localhost
is it the problem?