Any idea what this computer virus is/how to fix it?

A few things I've noticed so far:

1) Internet won't allow me to open links from search engines such as Google. Instead, it reroutes me to things like AskTheCrew.net and random sites such as an online lawn mower store. However, I can copy the URL offered from the search and paste it into the address bar to go to the site I wish to go to. This is obviously irritating.

2) I've attempted to change the desktop background and icons a few times and it works until I restart my computer, when it returns to the prior background.

3) I have Norton 360 on my computer, and every time I turn on my computer it warns me that I'm at risk and Norton has to do several scans and "fix now" several issues. Every single time - it will constantly tell me that. It also always tells me to upgrade to SONAR and when I tell it to fix it, nothing happens.

4) Occasionally, when I'm attempting to restart my computer, it will have some kind of Startup Scan going. It looks like a Microsoft Classic box and it runs sometimes just five minutes, but other times almost an hour. Afterwards, it always continues to start the computer and allow me to log in.

My sister has been having similar issues with her own computer, but for much longer than I have. Also, her computer will sometimes refuse to play sound or stereo at all.

If you could tell me perhaps what this virus may be and how to get rid of it because Norton Antivirus has done nothing to help, I'd be very appreciative. Thank you all!