If you are looking to run a test web server then you can most certainly install Apache/MySQL/PHP on a windows box. They are all "free" software. You can run them legally without paying the orginal developers.
If all you are doing is running test apps for your own education, then running these on your "primary" computer won't cause a problem. I have Apache/MySQL/PHP running on my laptop for testing purposes and there is no problem or performance issues at all.
If you are looking for more of a production environment I would recommend moving the web services to their own hardware. It doesn't have to be too powerful, but I would recommend as much RAM as you can reasonably afford.
As far as bandwidth consumption, that all depends on the scope of what you are doing. I ran a personal web server for about 2 years using my personal bandwidth and never saw an impact on throughput. Unless you are serving large files or hosting an ftp mirror most traffic is in small bursts.
Ultimately, it all comes down to what you are looking to do. But, to answer your orignal question (again) yes, you can turn your personal computer into a "server" to test web/sql practices.