How big is your psu? Do this happen when you are doing SD? If your 12v and 5v is more that 5% off then you will need a bigger PSU. Check your memory by going to task manager and under the performance tab look at your peak level under commit charge. Is it bigger than your physical memory total. If so by a lot then you will need more memory. Also make sure all non essential programs are off so it won't rob your ram capabilities.
If it's happening at exactly 42% each time, then that tells me there is something going on at that point in your footage. Try taking a 10 second portion of your footage out of the timeline, and render from there. If it makes it all the way through then you know where your problem is, if it doesn't then you might start checking out hardware issues.
With 2 gig of physical memory then most likely thats not the problem. Check to see if you have a program running in the background that interupts the rendering process.
Sounds like a codec issue. I'm not positive, but with WMV, Vegas finishes the first pass of a two-pass encoding before 50%... sounds like it's crashing at the very start of the second pass. I'm assuming you are trying two-pass encoding, right? Do you have alternative codecs or a codec pack installed? What version of the WMV codec are you using?
How old is your Hard Drive? Check to make sure you have dma enabled. You can go to start/controlpanel/system/hardware/devicemanager/ATA-IDE/primary channel.
Make sure it is dma and not P10. If so then go here to fix it. P10 makes the drive go slow and therefore you might get crashes since it can't keep up.