Load Balancing of Two WAN private IPs

Hi, I don't know if the subject is set right or no! here is what I have and what I intend to do!

I have a simple office which has 20 comps and they all have to use internet to and only plain simple internet no VPNs or any carp of that sort.

Now here is what I am doing right now I have one ISP cable and that comes on one computer which use ICS to distribute the internet to the other pcs.

Everything is fine everything works great but the catch is that my isp has a download limit..after which it charges on per mb basis. Well thats normal but I am a poor man...Ok now ... the solution I am using .. I got another ISP.. so when one ISP limit is done I use the other one.. here is the example

isp A -- gives 150mb download free per day --so if the speed of 256kbps is used the entire quota is downloaded within 2 1/2 hrs.

isp B-gives unlimited download free per day BUT the max speed is only

My requirement is to merge both isps into one isp A 150mb on 64kbps will work for 10 hrs and isp B unlimited download will be there for the entire day and I will get a speed of 96+64 kbps == that is roughly good as 256kbps.

Now both of the isps use the private ips at my end 10.0.*.* and

I am looking for a cheap solution ... platform win XP

Please advice..