Can't install software anymore

I have been running windows 7 professional since it came out. Ihas been great until about a few weeks ago. I have noticed a couple things.

1) when i click the start menu and start typing the name of a program is is slower and
usually finds documents instead of applications that i may be searching for.

2) more important - i cant seem to install software anymore. Any time i go to install software,or even install anupdate to something like itunes or windows live photo gallery, it goes through the install but whenits dont, the software doesn't seem to actually be installed. It will have an entry in the start menu but nothign is there. Like Live photo gallery has a generic icon instead of the correct app icon, and when i click on it it says "Windows cannot access the specified device, path or file. You may not have he appropriate permissions to access the item."

I had a heck of a time getting itunes to update to the latest version the other day. I had to install and uninstall it several times. When I tried to do a complete uninstall, part of the process was to go to program files and delete the itunes folder but when i tried, it said that i did not have permission to do so.

so lots of weird things seem to be happening, but the biggest problem is that i cant seem to install apps.

any ideas on how to resolve this?

I ran a complete systemscanwith Microsoft security essentials and it found and removed a couple items but that didn't seem to resolve anything.

Install Malwarebytes, update and run a Full Scan.

Run SFC /SCANNOW Command - System File Checker

Can you System Restore to before the problems began? If necessary run the Restore from the DVD or Repair CD: System Recovery Options

Study the Event Viewer>Custom>Admin View to google repeat errors to resolution. Also the Performance log on Advanced Tools page accessed by clicking your WEI score link at Computer>Properties. Look for cued issues at top of Tools page, Generate a System Health Report.

Type "Problems" in Start Search box to see if there are solution to problems sent or not.

Consider a Repair Install

These problems frequently crop up when 3rd party tweaks or optimizations are applied, which will always come back to bite you in Win7 since it is already a lean, perfectly balanced OS. You may need to Clean Reinstall to overcome the problems if the above doesn't help.