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Picaxe connection problems

I bought a PICAXE 20M for a new project, and I'm trying to download code to it over the serial port of my computer.  I set up the circuit with the 22k and 10k resistors and tried to download, but I got a 'memory validation failed' error.  In the options dialog, I can get the Firmware version with no problem.  After looking at the PICAXE forum, I did the following:

- Checked the voltage going into the PICAXE - at first it was reading slightly over 5V, but I added a 7805 and it now reads about 4.8V. 

- I tried doing the hard reset.

- I ran the COM port test from the options dialog.  The 'LED off' voltage at the SERIN pin reads correctly.  The 'LED on' voltage reads 5.56V.  Is this an indication that my serial port is putting out too high a voltage for the PICAXE to read?

 - I tried switching the 10K and 22K resistors.

None of these things solved the problem.  I read in a couple of the postings that using the USB cable solved this problem.  I'd rather not do that because I am on a very tight budget, but if that makes communication 100% reliable, I would do it.

 My questions are

- Is there anything else I can try to get the serial port to download code to the chip?

- In your experience, is the USB cable a much more reliable means of downloading code to the chip?

And while I'm here, I'd like to ask - can the SERIN and SEROUT be used to communicate with a serial device at TTL voltage levels (+5/-5V)?

 Any help would be greatly appreciated!

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The PC uses a much greater voltage range than TTL. An rs232 converter is required if you don`t have one. Are you using some kind of picaxe board or just the chip? I know nothing about picaxe but if you have some kind of development board it may have serial level converters already. If it doesn`t you could damage the chip.

4.8v is pretty low for a 5v regulator output. What voltage is the regulator input?

I was going by memory on the 7805 output voltage - it is closer to 4.9V.  The input voltage is about 9.5V. 

 Thanks for the info on the serial communications!