My PC slows down when trying to download

Hi there, first post here and i need a little help, i've been using bitcomet for some years already and never had a problem like this one.
Whenever i try to download a file that's 13+ GB my computer slows down a lot and the cpu gets to 30-100% usage, can still navigate but with a lot of lag.
I have already downloaded many times larger files, so i really don't have a clue what may be happening, i believe it also slows down a bit my download speed.

I'm using BitComet v. 1.37 64bit o Windows 7
I've also tried downloading the portable 1.37 version it gets a little better letting me navigate normally in my computer but still when i try to open bitcomet it lags a lot and i have no other choice than shut down the process.
I have not tried another version though

ADSL Modem but haven't done port forwarding since it has never slowed down my downloading speed.
I use ESET Smart Security 7.0.302.26

10mb Download speed
1mb upload speed
My computer:
Core 2 Duo @ 2.4Ghz
4GB RAM

Many Thanks

Ps. Sorry for my english, and long post, trying to make it as clear as possible!