General PC slowness and Game crashing.

Ok, so about 8 months ago, I bought a very expensive computer from Dell (XPS 630) and Up untill recently Ive been happy with it, I mean its had a few problems every now and then, but nothing too major, however Ive recently noticed that its started to generally feel a little slower, the computer seems to have a few moments where it will freeze up for a second or two and allot of the games I use have started to simply crash, for example today I was playing World In Conflict and after 6 minuites or so it simply just froze and went black although I could still hear the game playing in the background I couldnt minimize back onto it and it said it was not responding. For about two days my CPU usage went from 1 - 5% idle up to about 50% at idle, however I performed and error check on my Hard Drive and its now gone back to 1 - 5% usage, during games it dosent shoot up that much, however if Im rendering something it goes to 90%-100%. My Ram usage is generally at around 50-60%. I use Nod 32 and I defrag my computer often, Im going to open up the case shortly to make sure its not all cloged up with dust, but it shouldnt be as a I clean it like once a month or so. I'd really like some help on this, oh and a heads up, im going away on the 5th for 3 weeks so If i dont hear from anyone before I go, I look forward to hearing from you all when I get back .

My Specs are as follows....

Dell XPS 630 Intel® Core™ 2 Quad-Core Processor Q9550 (2.83Ghz, 1333MHz FSB, 12MB cache)

4096MB 800MHz Dual Channel DDR2 SDRAM [4x1024]

Hard Drive 1TB NCQ SATA (2x500GB)

7200Rpm Dual HDD Config Raid 0 Stripe

Single 1GB DDR3 nVidia® GeForce™ GTX285 graphics card

PCI Card Ageia PhysX 128MB

Integrated HDA 7.1 Dolby Digital Audio Operating System

Windows 7 32bit