Does my personal firewall on my laptop protect me against my company's
network packet sniffers used to monitor employee internet activity?

NO

Packet sniffers monitor ALL traffic on the segment they are sniffing. 
This means that everything is monitored.


Your personal firewall prevents *things* from directly connecting to
your computer without your permission.

The other comment is correct, a PC Firewall will only stop something
trying to connect directly to your PC. A sniffer monitors the traffic
at the switch level and sees it before it gets to your PC and directly
after is leaves your PC.

That said they probably aren't using a sniffer to monitor employee
internet activity, they are likely just using the corporate firewall
logs and a reporting tool along with the corporate proxy server logs
and a similar reporting tool. Again, in this instance your PC firewall
will do nothing to stop them from seeing your internet activity.

forge

Your personal firewall dosent protect you from sniffers.

You need to have a Active hubs, because it only send packets to the
intended machines.

But, another solution is to download one of this, or both to try it:

http://www.securiteam.com/tools/AntiSniff_-_find_sniffers_on_your_local_network.html
OR
http://www.securiteam.com/tools/Neped_-_Detect_sniffers_on_your_local_network.html

REALLY Nice apps.

Hope this helps. Any doubt just ask me.

Good Look.