I haven't used XAMPP yet, but if it's the same as Uniform Server there is a version of Sendmail you can install that will let you use your SMTP server to act the same as a true Sendmail.
This is what I use and it works.
http://www.glob.com.au/sendmail/sendmail.zip
sendmail.exe is a simple windows console application that emulates sendmail's "-t" option to deliver emails piped via stdin.
it is intended to ease running unix code that has /usr/lib/sendmail hardcoded
as an email delivery means.
it doesn't support deferred delivery, and requires an smtp server to perform
the actual delivery of the messages.
After siting it up in your testing server, edit your cart with your ISP SMTP information. Be careful, it will send outside your testing environment. also another heads up, you have to be online to mail, which means your testing server is exposed to the world unless you run a good firewall.