|There is no risk-free method to accomplish what you want.|
Before using any partitioning tools on your system you should back up all your important files to an external destination and be prepared to wipe the drive and re-install from scratch if something goes wrong.
There are any number of partitioning tools available for download and they are run from either a bootfloppy or bootable CD.
You would proceed by reducing the size of the E drive by whatever amount you want to increase the C.
Save and re-boot then proceed to expand the C into the space now freed up.
I have no experience with these type partitioning tools so am not in a position to recommend one over another, but the forums are full of threads created by people wanting help to recover their systems after re-sizing partitions went bad.
Be prepared for worst case.
Another option would be to un-install programs from the C drive and re-install them to the E drive to free up space on C and live with the current partition arrangement.
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