Hi,
And welcome!
There are some things you will want to do to run MODX on xampp. I don't know if windows 7 is too much of a factor actually. I have a number of installs running on xampp with windows 7, various versions. It IS slow, yet with 2.2.x it's sped up quite a bit.
See
http://bobsguides.com/xampp.html for information about xampp installs. There's some configurin' to do.
I believe you'll also have to enable cURL in \xampp\php\php.ini to get the package manager to work - (google cURL and xampp). Apache Friends has a list of file locations at the bottom of this page
http://www.apachefriends.org/en/xampp-windows.html
Also you'll want to verify that Apache, php, mySQL, and the manager's default_timezone settings (sytstem settings -> system and server) all have matching timezones set. See php.net/manual/en/timezones.php for a list of php timezones. Use the php timezone code for the MODX default_timezone setting. With a new xampp install you'll want to verify all the timezone settings throughout and set them for your location anyway.
If I can remember anything else I'll post again (it's been a while, sorry).