Hi there, just for a quick reference, what types of games are you attempting to play online? it sounds to me like you're having graphics card problems, are you using the vista generic drivers to run your card? or have you gone to the manufacturers website and retrieved the latest drivers for your card? if you have not i would highly recommend you try this first.
A second thought i'm having is are you connecting to the internet via router? if so, hit up PortForward.com - Free Help Setting up Your Router or Firewall
and make sure you've opened the ports on your router to allow your games to communicate properly via your pc > router > modem > WAN
A third thought, are you running any AV software which might be interfearing with the networking operations of your games? if so look into clearing that up, just something to be concious of
A fourth thought is maby UAC in vista is causing some undue problems that you're not aware of, for trying's sake shut it down (along with your av software for the moment) and try again. it's quick to do..... run > msconfig > the "tools" tab > scroll down to "disable UAC" > hit launch > restart computer
These suggestions are the obvious ones, if you've tried all four suggestions and arn't having any luck, then we'll get into the gritty work of figureing it out. But the simple stuff is always what you start out with when diagnosing a problem like this.
Btw what kind of graphics card are you running? Processor specs? Memory Dimm Specs? thanks