I set up a small network, using a network cable of 1 meter.
My PC (Windows XP Professional SP2) connects to the Internet, and shares this connection with the laptop (Windows XP Home SP2).
The laptop has no access to the PC, but the later does have access to the laptop.
When the laptop is on and is using the Internet sometimes, yet frequently, it makes my PC freeze.
It's really getting annoying, since my family won't stop using the laptop and then they complain about it and blame it on their new GMail account, or some stupid thing like that.
And most importantly, I can't download anything from anywhere when they're connected, because it freezes.
I know this is extremely vague. So vague that I can't find proper info when searching with Google.
But maybe you can help me find what is causing this...
Some software and configurations for the PC that can be useful to know:
Using Windows XP Firewall
Panda Titanium 2006 AV (firewall disabled)
I'll call the tech guy sometime this week, too...
Hope you can lend me a hand...
Update: It was dodgy Panda.
/ 
