Run a few scans to be sure you're not infected...
Once you've finished those scans, try running the installation again if they fixed any problems. If they didn't then you may have a problem with the operating system itself.
Open an elevated command prompt and run the following command:
(Start > Search > cmd.exe > Ctrl+Shift+Enter)
sfc /scannow
After that process finishes, restart your computer and then attempt to install the files again. If you're still having issues then you likely either have a corrupt installation or somehow your internet connection is corrupting your downloads.
If you think it may be your browser causing the corrupt downloads, you can try this method to download firefox:
Open an elevated command prompt
(Start > Search > cmd.exe > Ctrl+Shift+Enter)
Type the following commands in order, pressing enter after each one and waiting for the previous command to finish before starting the next.
FTP ftp.mozilla.org
anonymous
blank@email.com
HASH
cd /pub/mozilla.org/firefox/releases/latest/win32/en-US/
BINARY
lcd c:\
mget *.exe
yes
(Now wait for the # (hash marks) to stop.)
Note: Downloading through command prompt may prompt your firewall to ask for permission, just say yes
If you would like to do the same for opera, use the following commands:
ftp ftp.opera.com
anonymous
blank@email.com
HASH
cd /pub/opera/win/1053/en/
BINARY
lcd c:\
mget *.exe
no
yes