No offense taken, and yes i do know what NAT is and does. FYI 'a NAT' doesn't exist, but rather NAT is a function of a device. Any router performs NAT. Many DSL modems have a builtin router but only one physical ethernet port. Most people who have a DSL modem like this stack another COTS router (linksys for example) right behind it and just offset the local networks, aka 192.168.3.x for DSL <-> Router and have the Linksys Router use 192.168.1.x.
This works fine and dandy till you try to host anything from a computer on the .1.x network... where the two (+) layers of NAT start messing everything up.
To Fix, either:
A) Use DSL as router and shift the Linksys over to act as a simple HUB, using the DHCP & Port forwarding on the DSL modem/router.
-or-
B) Shift the DSL modem/router over to Bridge mode, have the Linksys aquire an IP from your ISP via PPPoE and use the DHCP & port forwarding on the Linksys.
Personally, I prefer (and recommend) B as typically the COTS routers will have 10x more stability, throughput and features than the craptastic DSL modem/routers that come standard issue with most broadband packages. Not to mention that you can Flash the Firmware of Linksys's WRT54G with the 'unsupported Firmware' and make it into one hell of a router!
....And it sounds like to me that someone has been visiting "not so on the up and up" type websites.

The only time I have ever got a nasty on my network is from visiting a warez or porn site. Most of these keyloggers and such are 'legalized' by the fact that
'the user clicked Install.... they didn't HAVE to"... which, yes, is a load of BS.
Thats why I have a VM that I fire up when I need to visit any sites that might be shady. If the VM gets a bug or the like, i just delete it and make a new one... zero data loss.
Oh yeah, I did some research a few years back and have come to find that most of these nasties that we get infected with originate from countries outside the US... and since there is no international Law, especially on the internet, then there isnt much legal recourse for anyone losing lots of data to a virus. I suppose this is why data recovery/backup centers can charge a premium and get away with it lol.