Ok so I just installed the Clean Installer and whenever I try to open it, I get this error:

Sims2Pack Clean Installer has encountered a problem and needs to close. We are sorry for the inconvenience.

I do have .net framework 2.0, 3.0 and 3.5 versions.

What could be the issue?

There's not enough information there to tell us what's wrong.

Try downloading, extracting and running the no-install version.

Ok I'll try that.

Edit: No luck. Still getting the same error

Edit 2:
I'm having the same issue with compressorizer and SimPE too.

Error signature:
AppName: simpe.exe AppVer: 0.72.1.33909 AppStamp:48d7e8eb
ModName: kernel32.dll ModVer: 5.1.2600.3541 ModStamp:49c4f751
fDebug: 0 Offset: 00012a6b

Okay so I read some stuff online which suggested to reinstall .net framework. I did it but now, I have a new error:

system.io.filenotfoundexception: Couldn't load file or assembly 'Accessibility, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a] or one its dependencies.

There's a lot more under the above mentioned error. Here's a picture:

This really looks like a .NET problem. Which operating system are you running? 32- or 64-bit? Did you get any errors during installation of the Clean Installer? Did you try the no-install version?

Thread can be closed. I fixed it. Thanks a lot Mootilda for your support, I really appreciate it.

Here's how I fixed it for those who might run into the same problem.

1. Download .net framework cleanup tool by Aaron Stebner
2. Open it and allow it to remove all the version of .net from your PC.
3. Now, install a fresh copy of .net. If you get an error 25015 about 'Assembly' not found (like I did), disable your anti-virus and try again but if the error persists, go to my computer > WINDOWS and look for a folder named assembly. Rename it to something like assembly_
4. Retry the installation again and it will automatically create a new assembly folder.
5. Try to run Clean Installer or other .net based program and see if it works. For me, it didn't. I got the error that I posted above. So after brainstorming for a while, I decided to uninstall .net again and reinstall it just to see if I'll encounter the same assembly error again. And voila! No error and the application ran perfectly. But do not touch the assembly folder during the second time around.

I am having problems with this too, except when I tried to download the newest version, it says I have an older version already installed. When I try to search for it, it does not appear anywhere. How can I go about fixing this please?

Try this:
http://www.modthesims.info/showthread.php?t=217787