Hard Drive checking utility: What's a good one?

[Elvis2]

I sold a buddy of mine a P4 system a few month's ago and it's been acting wierd lately. On boot, it would say HD not detected or unable to find OS (XP Home). He'd reformat and everything would be ok for awhile then start acting up again. He has two Maxtor D740X 40 gig drives in RAID 0 running off a Rocket Raid 100 controller card on a Gigabyte 8SQ800 mb. The latest incident happened yesterday when he went to d/l updates and the system froze. After he rebooted, he got a Windows file corruption error. Reformatted again and sofar, everything's green. I also has NAV 2004 installed but it sounds more like a hardware and not a virus issue. He paid me some good money for this system so I want to treat him right. I'm thinking about making the 500 mile round trip to his house and replace the RAID setup with a single 80 gig WD "SE" drive instead. I'd like to make sure it's a drive issue tho before I buy the new drive and make that trip.
Any ideas?
TIA