I think Norton is crapware
I used avg-free in many occasions and it works fine ...
I would suggest avg free as a better choice than Norton
http://free.avg.com/eu-en/free-antivirus-download
Run full scan and rootkit scan
Before doing reinstall, you can try SFC ....
SFC is a windows tool that actually verifies that the core windows executables are not messed us, at least it's what's being claimed that it should do
From an administrator command prompt run following: sfc /scannow
The problem with windows is not windows, but
- 3rd party programs that 'mess up'
- 3rd party drivers that 'mess up'
- infections coming via Internet based on the fact that 99% of the world run windows from an account with administrator privileges, this is by itself an architectural flaw, you should not do this ....
It's very very easy to protect yourself from this, just run Firefox, Chrome or IE from an account with only user privileges and with no write access to system files, it's then practically impossible for system to get infected via Firefox, Chrome or IE