Cleaning the log file is important! Other players will be trawling them looking for IPs they haven't hacked yet, and if yours is in there you'll end up with a virus! Of course, you can do the same.

Viruses on your virtual PC don't actually harm your ability to play in any way, but they are making money for that other player. Want to get rid of them? Find some anti-virus software in your hacking exploits!
Once you've got a few bot PCs sending out spam you can harvest some cash, stick it in your bank account (edit your own log file, the banks can be hacked too!) and upgrade your hardware with it.
I still haven't gotten up to being able to hack a bank yet: their firewall is better than my "waterwall." Shame too, I've got 4 bank numbers I've skimmed off log files.
