My computer takes ages to close down and I don't know why
Also, when I am on a page, and I open another page, it takes time to open, or it gives me an error "page cannot be displayed"... any ideas

Thanks for posting this as a question, least this way you get all of us having a go at it.

As has already been mentioned here's a list of things to keep your machine running well.

Have you made sure all your drivers and patches are up to date ?
Worth try are some other anti spyware/virus apps and some system maintenance.
(I'm going to assume that you are using XP even tho' you didn't mention.
Most of these steps will work with any Operating System)

Here's some simple steps for you to take.
1/ Remove temp files and other rubbish form your system, either with inbuilt Disc Clean up or CCleaner.
(Disc clean up; open my computer > right click your C: drive > properties > Disc clean up button on general tab. Let it run and select everything)
2/ Defrag; open my computer > right click your C: drive > properties > Tools tab > Defragament now button.
3/ Error checking; open my computer > right click your C: drive > properties > Tools tab > Top button > check now. Check BOTH boxes, OK the error message and reboot.
4/ Page file; Right click 'my computer' > properties > advanced tab > Performance settings button > advanced tab > Virtual memory (near the bottom). To make things easier for you select System managed size > click set > then OK's all the way out again. (I suggest 'system managed' as there is no consensus on min and max sizes to set)
5/ To make sure everything is running fine also run both anti virus and anti spyware apps (make sure that they are updated first ;)) (AVG is good and free AV)
(A couple of good removal tools are Spybot and Adaware)

ALso an on line virus and spyware scanner is Trend Housecall

I know you've already scanned but never hurt to be 110% sure ;)

That'll do for starters, keep you going a while.
If you culd let us know the system specs there maybe some others bits we can try.

Thanks will do that and let you know... and yes I am running XP