To be honest, this doesn't surprise me at all. There are a couple issues:

1. You've got your core clock pushed to the max, assuming you're on stock voltage. I run my 5830's at 960Mhz.

2. You're running on a minimally suitable power supply. I would want at least 500W for a single 5830. My 4x5830 uses a 1200W PSU.

3. Excel 2010 uses DirectX for rendering. If you're miner is putting your GPU at a 99% load, I would expect the driver to crash if you attempt any sort of rendering.