PC stuck momentarily when HDD works (read /write)

The problem I have is that constantly, when the hard drive works (ie the LED lights indicating you are reading / writing), the PC slows down momentarily. It is noted for example in the mouse (the cursor is "stuck" half a second, repeatedly for one or two seconds), or if I'm listening to music from an internet radio station, the music also gets "stuck", repeating a syllable for a moment (example: "la, la, la, and continues). The more you use the hard drive (ie copying a large file), the more acute the problem is.

All PC drivers are installed (at least the PC does not alert a driver is missing.) The PC discs operates with RAID1. The RAID1 seems to be well configured. I started the PC with both discs separately, but I have the same problem.


The PC has:
Windows XP (SP3).
ASUS P5K MotherBoard.
RAM: 4 Gigabytes.
Processor: Core 2 Duo E8400 3.00GHz, 1333FSB, L2: 6MB, 65W, rev.C0
BIOS: AMI Version: 0704.
HDD: two hard drives in RAID1, Seagate 7200.10, 500 GB, ST3500630AS, firmaware: 3.AAK, Device Type: HDD 3GNCQ
RAID Controller: I found two possible names for the RAID controller:
-> JMicron JMB36X version: 1.17.15.0 (03/24/2007)
-> JRAID SCSI Disk Device 5.1.2535.0 (07/01/2001)
Moreover, each HD driver is: Intel (R) ICH9 2 port Serial ATA Storage Controller 1 to 2921.

What do you think the problem is? What may be happening and how do you think I can solve it?
Thank you very much.