Possible bad memory
You didn't state the memory brand, but I built a brand new S1689 with everything new, and it went ballistic immediately. I had similar problems, and I was fortunate enough to have another spare 512mb memory chip laying around. Turns out the brand new (Ultra) memory chip was defective. It did show up under memtest though.
If you are running winxp, you may try another os like win2k. It may be the ide drivers for the hd are flaking out in xp. XP has a mind of it's own when it comes to drivers and doesn't always choose the correct driver, but usually does for the most part.
MAKE SURE YOU PUT THE Single Sided memory in the blue slots, double sided memory in the black slots - VERY IMPORTANT! Use default mb settings for now.
Make sure you have the latest bios update (I think they only ever put out one), and make sure you have the correct motherboard chipset drivers loaded. Other than that, if it exhibits the same behavior under win2k, return the mb under warranty. Don't kill yourself trying to figure it out. Bad mb's can be tricky to spot sometimes. I have access to a hard disk duplicator that checks for disk defects, so I have the luxary of running hardware diagnostics to check the disk another way. I have enough computers laying around that I can easily move the hd, memory and peripherals to another mb to isolate the problem. My S1689 runs rock solid with 1.5gb of memory under win2k. I am quite please for the low cost and I am not easily pleased. I am a bit disappointed that I can't put the same memory in all 4 slots, so I went to bigger chips. Just for giggles, you may try increasing the swap space to 1gb in virtual memory settings. Look for core files (a .dmp file). If you find a core dump, go to microsoft and download the debugger tool, and open the .dmp file in the utility. The stack trace will tell you what was executing in memory that caused the core dump. I traced a bad printer driver from Minolta on this motherboard. I fought with Minolta until I got an engineer to concede the driver didn't work on ALL motherboards. They gave me a "unidriver" and viola, the printer driver worked! The printer driver worked on my asus mb, but refused to work on the S1689. I had to use their unidriver, which was not on their support site, but I got it emailed from the engineer after I beat on them enough. I don't give up easily! The stack trace told me the problem, but the 2nd problem was getting somebody in 2nd level support at Minolta to understand my stack trace proved their driver was at fault. I have found debugging a core dump is quite valuable in pointing the finger at the cause of crashes, corruption, blue screen of death, etc.
I am an ASUS diehard, and tried this crappy mb for giggles. I was impressed for the price and it has run great for the last year. Good luck. I'm sure you'll find the problem, eventually!
Almost forgot, if you go to win2k on a 250gb disk, the initial load will only give you 138gb ( a sp3 and above issue concerning LBA addressing). Not to worry. Just grab 100gb for now for the primary partition and do your troubleshooting to isolate the operating system as the culprit. If it works, there is a registry hack to get the rest of the disk back, but I am not happy with the issue itself. It's like the old NT4.0 8gb limit issue. Thanks Microsoft.