IMac always freezing, coloured wheel etc

Question

Hello
I have noticed, for some time that if I have more than one window open in Safari, the iMac tends to start freezing, ie instead of the arrow you get the coloured wheel and cannot continue until it unfreezes.
Now, I am noticing that, even if only one window is open, the same thing is happening, eg in myspace I cannot even scroll down the page without it stopping and starting.
I don't know what is required in terms of routine maintenance to keep the system running properly.
In addition, today, I had a pop-up which said the system is affected by viruses. Obviously, I don't know if this was something genuine or malicious. I agreed for it to scan, and it said 527 threats found.
It then asked me to download Antivirus 360, which I did not agree to.
Because this freezing happens typically when more than one window is open, I assumed it was a limitation in the iMac rather than broadband-related (I have basic Talk Talk broadband). I now have to consider whether it is virus-related.
I have never installed anti-virus software, other than what is in the Apple software updates.
Can anyone suggest what I should do to get the iMac running more efficiently, and if virus infection may be the cause, what is the best remedy?
I would appreciate any advice
Thank you
notanthony

Answer 1

Miriam
Thanks very much
I restarted from the install disc (hold down letter c), and clicked repair for the Mac HD.
At the end, it said - No repairs were necessary.
I then restarted from the internal drive and followed the procedure to repair permissions.
Below, I have pasted the result.
I don't know yet if it has been advantageous, but anything very good or very bad, I will post again
Thanks very much
notanthony
Repairing permissions for “Macintosh HD”
Determining correct file permissions.
Permissions differ on ./Applications/iTunes.app/Contents/Frameworks/InternetUtilities.bundle/Contents /MacOS/InternetUtilities, should be -rw-rw-r-- , they are -rwxrwxr-x
Owner and group corrected on ./Applications/iTunes.app/Contents/Frameworks/InternetUtilities.bundle/Contents /MacOS/InternetUtilities
Permissions corrected on ./Applications/iTunes.app/Contents/Frameworks/InternetUtilities.bundle/Contents /MacOS/InternetUtilities
Permissions differ on ./Library/Application Support/Macromedia/Shockwave 10/Shockwave 10 Preferences, should be -rwxrwxr-x , they are -rw-rw-r--
Owner and group corrected on ./Library/Application Support/Macromedia/Shockwave 10/Shockwave 10 Preferences
Permissions corrected on ./Library/Application Support/Macromedia/Shockwave 10/Shockwave 10 Preferences
Group differs on ./private/etc/authorization, should be 80, group is 0
Owner and group corrected on ./private/etc/authorization
Permissions corrected on ./private/etc/authorization
Permissions differ on ./private/var/log/secure.log, should be -rw------- , they are -rw-r-----
Owner and group corrected on ./private/var/log/secure.log
Permissions corrected on ./private/var/log/secure.log
Permissions repair complete
The privileges have been verified or repaired on the selected volume