Need to sort this out :/

I have an extremely annoying issue. I've asked on here before but I've never solved it.

Basically I've just extracted a 8.3gb file from my 320 drive to my windows drive. During this my PC came to a crawl. The mouse would pause, applications would just stick (no errors, just extreme lag).

It's really annoying and I have no idea if this is normal.

System:

Gigabyte DS3 965P
Q6600 @ 3200mhz
2*2gb Corsair 6400
XFI
8800GTX
250gb
320gb*2
Windows Vista X64 SP1 (had the issue on XP as well)

Someone on OCUK said that they had the same issue with a Gigabyte board and just moved sata connections. I cannot do this as the Jmicron ports are buggy and I've filled the ICH8 slots. Would getting a SATA addin card help or should I just buy a new board?