Justin,
Does Sony Vegas freeze or does the whole computer lock up, requiring a reboot? If the whole think locks up, then it may be a heat issue. Your “fairly old and slow computer” could be dirtier than it was a year ago. Making it work so hard can cause it to get too hot.
If so, a good cleaning will help. Otherwise I would need more info, like error logs, etc.
Jeff
2-Xeon X5680 @ 3.33, EVGA SR-2 Mobo, 48GB DDR3, GTX 580 3072MB, 16TB Attached Storage, Win7, Vegas 11 x64