Vista - Can't download using any web browser

For some time now, I have been unable to download. When I am using Firefox (3.0.11) , if I select "run this file" once the download has finished the program opens and comes up with an error message saying that the specified file cannot be found. If I save the file to a chosen location, the file is then not in this location when I search for it, and I cannot click on the file or select "Open file location" in the dowload window in firefox.

If i use Internet explorer (8) or google chrome, exactly the same thing happens.

If i receive an e-mail attachment (using windows live mail) and try and save the attachment, windows live mail either comes up with an error saying i don't have enough disk space (I have 86GB free) or says it has saved the attachment, but when I go to the specified folder, the attachment is not there.

If I try and download a torrent, using utorrent, utorrent comes up with an error message "utorrent cannot save the torrent to C:\users\<user>\AppData\Roaming\utorrent....

I read on this forum somewhere that creating a new user account may solve this problem, so I have done that, but it makes no difference.

I also read that it may be a problem with avg 8, so i should try unistalling it, however, when I just tried to unistall avg 8, windows informed me that the program had already been uninstalled. I obviously have not unistalled avg 8 before, so I am not really sure what is going on.... does anybody have any suggestions??

Thank you

very hard to say a solution! are you using a download manager? are you using a proxy for web surfing?
if you use a proxy for surfing,please config your internet download manager application for using proxy server.

if you need a free antivirus, use comodo.

I am not using a download manager or a proxy.I have a feeling that these problems first happened after I downloaded a windows update, so I am wondering if doing a system restore would help this problem?

Thank you for your help

It may be worth trying to do a system restore to the point before making htis update. If it does not work you should be able to revert back.