Yours seems to be particularly prone to problems with drive letter conflicts. I don't own one, but this shows up in a search.
Just to start from scratch..... do the un-install and re-install thing.
1. Open up Computer Management (right click on My Computer and select Manage)
2. Open up Device Manager and you should see an object called Portable Devices
3. Expand that and you should have 1 drive letter there.
*** If you are seeing Unknown Devices, then more than likely the drivers havent been installed properly***
4. Right Click and Delete it. If you are seeing 2 drive letters, just delete them both and Windows should do the rest.
5. Do a Scan For Hardware Changes.
> check if HTC Sync Manager starts to connect. If Not, keep following below
5. Take note of that drive letter in Portable Devices and go into Disk Management.
6. Have a look if that drive letter is being taken by another device.
7. If the drivers are installed correctly, you should at least see that drive letter as a Removable Device.
8. Right click on that drive letter, select 'Change Drive Letters and paths' and give it a new letter.
> check if HTC Sync Manager starts to connect. If Not, keep following below
9. If you delete the driver letters and Windows doesn't configure the drivers automatically when you scan for hardware changes... then you probably have Windows issues.
10. Run CHKDSK, ScanDisk and probably a virus scan and Malware scan.