Yes, you can peacefully coexist with Microsoft’s veritable Frankenstein , Windows Media Player.
Connnect your trusty Sansa and select the Sync tab. You can then look over the sync list (results). See the listing of files transferred to the device? With the Sansa connected, you can delete files from the library (PC) or the connected device, and WiMP will remember which end of pair keeps the file.
This is a little different from the days of HotSync and a PDA, where you had a choice of having the PC or the portable device as the master. With WiMP, as long as you make your deletions with them paired, successive sync sessions shouldn’t zap your files.
If the music is deleted separate from WiMP (as in using Windows Explorer), it may behave a bit differently.
I have a whole gaggle of Sansas paired with WiMP on three computers, and thankfully, never have had phantom deletions of files. Now, WiMP has killed itself on numerous occasions, requiring a rebuild of the DRM cache and also the database (the rebuild is a sight to behold, taking a little while, and can be interesting to watch).
I still like the WiMP11 interface better than 12 (Windows 7), since it’s more comfortable to work with. I’m still tweaking '12 in hopes of eventually mastering it.