When SEP client is installed it should reserve C: drive space needed for it to successfully distribute definitions to the client. As when C: drive falls below about 1GB of space the defintions stop updating.
This could lead to if a zero day attack was discovered and malicious activity could fill up the drive to prevent symantec from updating to protect against it allowing malicious software to get onto another driver or the remaining C: drive space and would never be detected until C: drive space was free'd up to allow defs to update and detect. This creates a potential risk in the product.
If when installed SEP reserved it's pwn C: drive space that couldn't be written over or to it would always have enough space to download, extract and distribute defs being a more effective product and preventing lots of machines on big estates from going out of date.