My computer tells me I have visited 178 porn sites - and it requests I download an installation to remove these siters and make them not visible to others. However when I try to down load this piece of software my avast anti virus thing comes up telling me that I cannot downloadf it. I have not visted any of the sites in question and if somebody has, they have deleted it from my computer

Sounds like a classic spyware infection!
Time to do some serious maintenance on your machine:

Here's my usual C&P for cases like this:


When was the last time you did some serious maintenance on your system ?

System maintenance includes:
Defrag
FULL patching
Virus scanning
Spyware scanning
removal of rubish files

Defrag is an inbuilt function in XP.
Open My Computer > Right click C: > Tools Tab and its there.

To make sure everything is running fine run both anti virus and anti spyware apps in normal AND safe modes. (make sure that they are updated first ! ;)) (AVG (http://free.grisoft.com/doc/1) is good and free AV)
(A couple of good removal tools are Spybot (http://www.safer-networking.org/) and Adaware (http://www.lavasoftusa.com/software/adaware/))

ALso an on line virus and spyware scanner is Trend Housecall (http://housecall.trendmicro.com/)

Just a note: actively running two AV's on one machine can cause problems.
So if you are thinking about it make sure your current one is disabled first.
Same thing applies to online scanners as well.

Removal of junk files is easy with CCleaner (http://www.ccleaner.com) a free app that does exactly what

If all this fails then a repair install may be in order:

Here's (http://www.microsoft.com/windowsxp/using/helpandsupport/learnmore/tips/doug92.mspx) the official Micro$oft way of doing it,
and Here's (http://www.geekstogo.com/forum/index.php?showtopic=138) one with screen shots.

Basically this installs windows over the top of your current setup.
So you don't loose any information you already have.

This lot should resolve your issue.

You shouldn't download the software because it can be harmful to your PC

Don't believe any popup on your computer that isn't generated by a program you know. As said above this is spyware (a small program that makes these popups) in this case this particular spyware program is trying to trick you into downloading a virus.

Run a spyware cleaner and just close popups from now on no matter how urgent they make it seem.