Getting error code "0x80070643" when try

Hello,
I had the old messenger version and went to sign in the other day and it told me I had to install the newer version to continue. I didn't want to, as I liked the old version, but it would not sign me in unless I did. so I clicked to install the new version. it would not install no matter how many times I tried. so I uninstalled messenger completely from my computer and tried again. this is the message I keep getting: An error occured during the installation of assembly. Microsoft.VC90.CRT, version= "9.0.21022.8", publicke. Please refer to Help and Support for more information. Please close all other open programs, and then try again. It looks like another program is preventing the installer from working . hr:0x80070643
So I clicked on help and support and that didnt' help because it told me to close any other open programs. I had no other programs running on my pc, no windows open , nothing. I don't know why I can't download messenger. My operating system is Windows Vista basic. Please help me figure this out. in the next question below you are asking me to tell you which version of messenger I am running...i uninstalled it so have none at the moment. I went to your website and clicked on dowload msn messenger, and that is where i'm having problems.i didn't know what to write, so I wrote MSN Messenger [All Versions].where you ask me about my connection to internet, I put cellular/wireless modem, it is just a wireless modem from my telephone company in the UAE (Etisalat)...no cell phone involved. I am on a laptop pc.
thanks