Some people have crashing issues, however the main problems come from playing with Firewall on or Router improperly configured. Some people have either put the machine into DMZ to play or bypassed the router all together..
Ugly things happen when you have a software firewall and Router on at the same time.. this is because the Router is a hardware firewall.. so you effectively have a firewall blocking a firewall..
Make sure Windows Firewall Service is disabled (go to Start, click RUN, tyoe in services.msc, click OK)
Scroll down to Universal Plug and Play and disable the service (right click service and select properties, set selection box to Disable)
then scroll down some more and do it again to Windows Firewall
restart system for changes to take effect.
Also make sure that your external IP address matches your internal IP address if you are using a router.. if the IP's don't match, you will crash out of missions.
Also make sure your Anti-Virus is turned off when playing .. AV can cause packet loss which will crash missions. This goes for all Anti-Virus scanners, however Norton and McAfee are the worst (these 2 are listed by PC Magazine to actually be Viruses in disguise since they are so system invasive and slows the PC down by 30%)