Dont know if this is going to help or not..........
Some of the following in actually around in old posts of mine from a couple of years ago, my problem related to PK3 but I think you have the same issue with your PK2.
In my case when switching chips from say 18f to 24f the PK3 needs to download different drivers for the pk3 from microchip, it did this every time I switched.
Now before you go 'but I am not having that problem' read on.
To cut a very long story short, I ended up bricking 3 pk3's (microchip replaced them free after failing to find a solution) the problem is WIN7 and the 64bit version, the drivers that the pk2 and pk3 use are windows drivers and not completely microchips, Microchip were really funny with me when my second pk3 went on a different machine that had win7 64 bit on it, I tried to see what was going on and 'I THINK' but have no proof that the drivers are constantly polling, on some machines with powered hubs etc this is ok, on other machines with weaker usb supplies this seems to cause the usb to almost reach brownout.
In the case of the PK3 the reason it was bricking (check way back on MC forums as there are a few posts on this there) was near the middle of the download the usb would drop the connection while the firmware was written to the pk3, this left it totaly bricked.
As the pk2 uses the same driver for all chips you dont get this trouble, but instead I think what you see is all the usb ports that are connected are constantly polling or whatever and draining the usb supply, obviously the less power hungry the usb device the less trouble.
MC ended up giving me a ICD3 as this uses one driver for everything and its a MC driver not a MS one. I saw these problems on 2 different laptops and 2 different pc's, the only common denominator was win7 64 bit, I never had an issue with win7 32 bit
LG
I hope this helps