How to scan slides on HP Scanjet G3010?

Hate to say it, but in my experience flatbed scanners do a terrible job with transparent media like slides and film. I suspect that your scanner may lack the dynamic range to work with a high-contrast media like slide film. Consider using VueScan, which is a low-cost but very high-quality piece of scanning software; I’ve had good luck getting better results with VueScan than with the software that shipped with my scanner. I think you can download and demo the software for free so you can see if you’d get better results before making the investment.

Other than that, you might be able to compensate for limited dynamic range by doing a process similar to that for creating HDR digital photos (bracket your scans and combine them using Photoship CS2 and above or Photomatix or something like that). Note that this process would be achingly tedious for 300 slides. There are lots of good articles on the Internet about the process of creating HDR images.