Usually, every time I download, my speed is ranging only from 30 to 45kbs, and very very seldom 70 to 80kbps...Is this normal?
Yes it's normal... the speed depends on the swarm speed, that's the average speed of each peer, if you can notice it in the torrent stats it would be close to your download speed.
The speed problem is caused when the number of requesters called peers or leechers is higher than the number of fulfiller's called seeders. usually if their numbers balances the speeds improve. I got speeds close to my internet plan limit but only on some torrents. Another problems is that most ISP's offer plans with upload speeds limited to a 10th of the download speed, so for my plan I can dl with 3 Mbps but upload with only 512 Kbps (64 KB/s). If every user is limited in such way expect 100 ppl to be able to upload with only 6.25 MB/s... split that to 1000 leechers and you get only 6.4 KB/s for each leecher. Once a piece is finished that leecher starts to upload to and helps the swarm.
I also replace the value 0 to 20 in upload speed (base on my upload test results)
That's not the way you should use that... anyway, that test measures your real connection, so the results are pure informal, nothing more. The value 0 should be set for the upload speed cause bandwidth test aren't always accurate and it hurts the swarms you dl from by limiting the amount of data you can give to them in a certain period of time. So I suggest to use 0 instead.
I also patch the net_maxhalfopen and set 80
I can guess what that means... you mean you patched the file tcpip.sys to increase the concurrent number of connections from 10 to 80 (or as much as you need) ? if not, Google: Event ID 4226 Patcher
still much persevering using utorrent just to download my favorite movies
Keep uTorrent... changing to another client won't solve the problem.