"Ports found to be OPEN were: 21, 23, 80"

Holy Crap!!!

21 = FTP
23 = Telnet
80 = HTTP

If I knew your IP address (which bad people will find soon enough by randomly scanning a range of ip addresses) - I could open a command prompt on your pc by the telnet command - put whatever files I want to on your hard drive via the ftp command - and run a web server on Port 80.

You're screwed dude.

Did you mean to run these services (http/ftp/telnet) on your PC?
Try: (Your PC) to see if a web page is displayed.

Since port 135, 137, and 139 are not open - I suspect it's your router/modem instead of your PC reporting these open port values.

Maybe it is the router supplied by your ISP running these and I would contact them ASAP to close the ports. Ask them if you can replace their router with a router of your own like a linksys.