This sounds like either 1) filesystem corruption, or 2) some networking anomaly where packets are getting corrupted (e.g. the .msi file contains broken/bad data).
Before doing any of the below things I recommend, why don't we compare checksums of your downloaded .msi file and the one I have which is known to work 100% correctly? This is the correct checksum of the installer, calculated using md5deep and sha1deep (and before anyone asks -- yes, different hashing algorithms output different values, so you need to make sure both people are using the same software!):
D:\downloads>md5deep "Intel SSD Toolbox - v2.0.2.000.msi"
835e99ae3a3f80291b965072a827869c D:\downloads\Intel SSD Toolbox - v2.0.2.000.msi
D:\downloads>sha1deep "Intel SSD Toolbox - v2.0.2.000.msi"
fd1971e9a036a7f499a9802b507f00f669676660 D:\downloads\Intel SSD Toolbox - v2.0.2.000.msi
If your checksums don't match what mine do then either your download is corrupt, you have a filesystem problem, or something really bad is going on with your C: drive (SSD? Unknown). I can assure you the Intel servers have the correct checksums (I've re-download the binary a couple times to make sure). If you want me to put up the Intel SSD Toolbox binary somewhere else so you can get at it (rather than from Intel's servers) I can do that -- the reason I'm offering this is that some ISPs use transparent caching HTTP proxies that are badly broken and give their customers bad/corrupt results (really, this isn't nonsense!).
Otherwise, I would recommend starting with running CHKDSK /F C: (run a Command prompt as Administrator, then issue that command), and when asked to reboot say yes. If you can install something like HD Tune Pro (trial version is fine), run it, select your SSD (again, assuming your SSD is your C: drive), click the Health tab, resize the window so I can see all the SMART attributes, then provide a screen shot of the attributes that would also be helpful (we can rule out any potential issues using that).
My checksums are the same, checked with HashTab, both MD5 and SHA-1 match.
Just to remove any doubt about path / rights / file integrity i am going to make a completely new directory elsewhere, redownload the install package, verify the checksums, and post the result. brb.
OK: it went through now. The only problem is, i have no idea as to *why*. In both cases i and my group (Administrators) have full rights to the whole directory tree and the file itself, and i am personally the owner of the file and the directory tree. Checksums match.
The only differences are that in unsuccessful case, the dir tree is deeper, and my rights are inherited from upper tree. With new successful try, the file is immediately at C:\tmp2 and given explicit rights (not inherited). Any other difference i cannot find.
But most important is that it finally works, thank you all so much
Edit: another difference is that previously i did not have Intel Rapid Storage Technology driver/software installed, but now have. Could it have been about this? Because the rights/ownership/file integrity aspect is really rather strange, as far as i can see from Security options and hashes the files are identical in every possible way.