Need help- Can't install any programs!

Hi, I was wondering if anyone would be able to help me with my big problem. I recently bought a used but referbished computer and after I tried it out and when to download a program, the program once downloaded would not install. The UAC control would come up on some programs then go away after pressing ok then nothing would happen, no installation box, not even in the task manager. I have already tried to install with the compatibility option but still no luck. The only thing that has installed was a system care program only because my friend sent it to me after downloading it from a torrent.

Any help would be appreciated.

Thanks for the quick reply,
System Specs: Intel Pentium 4 2.53GHz processor
1GB ram
Apparantly 512mb nvidia geforce graphics card (not sure though as it is not showing up)

32bit Windows 7 bought with already installed on the desktop PC without the windows disks

Just basic everyday programs like itunes, winrar, messenger and so on are not installing, the only way I can get them to install is to get them off my laptop via sharing the files on a public folder and installing them from there.

There are absolutly no messages that pop up when I try to install them. All that happens is I click, the computer thinks and nothing at all happens.

Also, maybe you can help me with another thing, random letters, words, symbols and all sorts of things keep appearing when I load a webpage but when I refresh the page it disappears.