Infected or no - massive uploads and Norton, etc.

I have a system, Windows XP Home, connected to the internet (DSL) with a Linksys BEFSR41 router. This system is expriencing slow operations but Norton has not detected anything. The system was recently reformatted and installed due to the same issue. Looking at Norton's connections report I've seen 3 instances of huge uploads when I'm not around. The first I caught (an upload of Windows 2000 install folders). These second and third instances I cannot locate. The details are:

2/4 11:37:41pm
(192.168.1.100)
Local port 3786
remote IP 192.168.1.101
remote port netbios-ssn(139)
bytes sent 72
bytes received 652340538
connection time 34:08.218

2/5 12:12:21am
(192.168.1.100)
local port 3788
remote IP 192.168.1.102
remote port netbios-ssn(139)
bytes sent 72
bytes received 1662800095
connection time 50:01.359

I have 3 systems attached to this router, 2 which I use to access the web and which exhibit slow processing. I've run an online scan using Kaspersky, with clean results. I'm not saavy in this realm so I don't know where else to look or if I'm just being paranoid.

Thanks.

 

I take that back. My third system also has this:

2/5 12:17:48am
local IP address 192.168.1.102
local port netbios-ssn(139)
remote IP 192.168.100
remote port 3788
bytes sent 8
bytes received 80251123
connection time 50:01.415