http://www.nirsoft.net/utils/cports.html
give this a try.
you should have around 50 connections on a typical day. most will just be set to wait.
look in the list and you should see about 25 services, 2 wininit, 5 wmpnetwk a couple for your anti virus, 2 for lsass and about 10 for system
if your running a game while running cports you will also see the game in the list and connections will vary depending on the game.
anything thats unknown do a check on. such as checking remote ip's that have established connections.
anything thats wait/wait close you can ignore as they will time out and disapear in 2 or so mins. but open connections should be looked at further.
while you run cports try running process explorer to see if theres anything running that shouldnt be.
http://technet.microsoft.com/en-gb/sysinternals/bb896653.aspx
compare the results to cports and close anything that you think may be an issue. (will take a bit of trial and error)
if theres something then check your firewall settings and block the connections of the suspicious apps.