back in the XP days, I used a software called “Unlocker” just for this problem. It probably still exists, I don’t know, because since Windows 7, the easiest way to find out what process locks a file is to open Resource Monitor (Start search: resmon) and on the CPU tab, using the “Associated handles” list, you can search for the file name and see the process in question (and kill it).
So yeah, Resource Monitor is a useful tool on Windows.
I remember when carriers not just locked the phones, they also had custom firmwares filled with bloat and customized skins and even locking down features and all that shit. For example, I had a Sony Ericsson K700i and it had a disgustingly customized FW on it, and aside that it was ugly, I could only play MP3 files that I purchased through Vodafone. Sending them via bluetooth (or even with IR) didn’t work, the phone refused to play it back.
(Then of course I found out that Sony Ericssons were pretty moddable phones so I replaced the FW with an original one and that solved all my problems. For some reasons, the fact that I patched the FW with countless of VKP patches and even unlocking it with a patch, didn’t void my warranty so whenever I fucked up the FW beyond my abilities to repair it or simply bricked it, I just sent it to Vodafone and they fixed it.)
And they did this even when Android became a thing. (Though, it was a Vodafone branded phone so… it was sort of OK. (technically it was a Huawei though, also pretty moddable phone))