Before I ask for an invitation for any of the torrent sites, I want to ask:
1. Is downloading virus possible? (I have a virus software, but just curious if anyone filters the site).
Anytime you download a file from someone unkown to you there is a possiblity of getting a virus. But like everything life, it's something that can be avoided by where you go and what you do. If you are looking for porn, or movies, or music (things that are very popular and sought after), you can definitely get a virus. Rumour is that the RIAA is putting some poison pill type files for download. But tennis matches are such a small segment of the downloading population it's very unlikely you will get one on purpose. Maybe if the person offering the download has a virus, unbeknownst to him or her. I've been torrenting tennis matches for a couple years with no problems.
2. I designate a folder in my HD with all the matches that I will offer for download?
Yes you do. But this is specific to the program you use to download torrents. Some are safer than others. Best to get a variety of opinions on what people use, and choose wisely. For what it's worth, I've used BitLord and Azuras with no problems.
3. Does my computer have to be ON all the time... and if not, then they can't download when it's off?
If you want to gain points you need to allow people access to the matches you download. The only way to do that is to have your computer on, and connected to the internet. So yes.
4. When someone is uploading a match from my PC, do I get a message, and does it slow my PC?
It will definitely slow your internet experience down. You can throttle it, or stop it if it's cause slow browsing. That's what I do. As for your computer performance, see question 2. It depends on the program you choose.