I want to be able to tell how many keys per second, using RSA 1024-bit keys, can be checked on a standard Pentium 4 system. How can I use this to determine decryption performance, and possibly remaining time?
It depends. Think of your IP address as the same kinda thing as a real address. If a criminal knows the address of a bank, what can they do? It completely depends on what security is in place.

If you've got a firewall running (e.g. Windows Firewall) or are behind a NAT router, you're probably safe. Both of these will prevent arbitrary incoming traffic from hitting your computer. This stops most remote exploits.

My suggestions:


Enable Windows firewall, or whatever firewall is available on your OS of choice.
Keep up to date with patches for your OS. These are critical!
Keep up to date with patches for your browser and any plugins (e.g. Flash)
Keep up to date with patches for your applications (e.g. Office, Adobe PDF, etc.)
If you're running any internet-facing services (e.g. httpd) on your machine, keep those up to date and configure their security appropriately.
Install a basic AV package if you're really worried. Microsoft Security Essentials (MSE) is a great choice for Windows, because it's free, unintrusive and not much of a performance hog.

