Grimjier,
I wrote this part of the installer, so I can give you some assistance here.
Google SketchUp Pro (7.0 and 7.1) require that you have the .NET Framework 2.0 (or newer) installed on your machine. The Free version of Google SketchUp does not have this requirement.
The SU Pro installer is configured to check for the .NET Framework and download it if you do not already have it. This only affects Windows XP users because Vista comes with the necessary version already installed. If the installer is attempting to download .NET Framework, it is because the program we use to check for its presence, which is provided by Microsoft, says you don't have it.
The download should normally be pretty fast, but in some cases, it may take a few hours to complete, or may just hang and never finish. Unfortunately, we use a Microsoft-provided mechanism for this entire operation, from checking for the product, to displaying the EULA, to downloading and installing it. Since it's not ours, there is nothing we can do to improve the reliability of the check, the robustness of the downloader, or the verbosity of the progress indicator. Believe me, I've tried.
Thankfully, I believe there is a workaround. You just need to install the .NET Framework 2.0 or newer before running the SketchUp installer and all will be well.
If you believe .NET 2.0 is already installed, then you would want to first uninstall it from the Add/Remove Programs menu, and then reinstall it. I think the program that looks for .NET bases its discovery on certain registry keys. If for some reason the registry has become corrupted or the keys are absent, that could cause it to want to download, even though you installed it previously.
The easiest and most highly recommended way to get .NET Framework 2.0 is via Windows Update. Optionally, you can either install .NET Framework 2.0, or something newer, like 3.5.
If you have trouble with Windows Update, you can download the .NET Framework directly. Be aware that the download links are different for 32-bit vs. 64-bit versions of Windows XP.
.NET Framework 2.0 32-bit: http://www.microsoft.com/downloads/details.aspx?FamilyID=0856EACB-4362-4B0D-8EDD-AAB15C5E04F5&displaylang=en
.NET Framework 2.0 64-bit: http://www.microsoft.com/downloads/details.aspx?familyid=B44A0000-ACF8-4FA1-AFFB-40E78D788B00&displaylang=en
Good luck and please let me know if this works.