I started mining with P2Pool / cgminer.
bitcoin-qt.exe is running. run_p2pool.exe is running. cgminer is running.
But all I get are rejects.
What might be the problem?
This might mean your internet connection is too slow. With p2pool, you are basically competing against all the other miners for internet latency. If you have bandwidth-intensive software running (downloads, uploads) try stopping it.