Bandwidth Upload and Download shared?

I am assuming your on DSL or cable in which case no, your upload rate dose not affect your download rate as they run on two seperate frequency?s so you can be downloading and uploading at full speed on both without them effecting each other, but youmust bear in mind that the website/location you are downloading/uploading to can control the speed of your download/upload depending on their available bandwidth.
Hope this helps

Answer 1 is correct in theory. However, how a ISP controls and caps your bandwidth can be another story.
Take Verizon for example, you can be downloading a file at normal transfer rate, but as soon as you start uploading a file, you can watch your download speed drop significantly. So you may want to check with your ISP.

Explanation to #2. Why it happening:

Each TCP Session has bi-directional traffic no matter what you are doing. TCP traffic has a thing named "window size" and "acknowledgement". Every 2-3 packets you downloaded, your PC should acknowledge reception (send confirmation). And if your "uplink" is congested, your confirmations will be delayed, and sender will wait for acknowledgement before sending another portion of packets. So, I would reccomend you to limit upload speed for 70-80 % of your link capability.

Good luck,