Most computer manufacturers include the RAM module configuration as they're selling the computer. Although your experience is unpleasant and frustrating, a little bit of investigative work probably would have turned up the default configuration for the notebook's memory slots.
If the computer is SUPPOSED to have a single 512MB module, then I would contact HP immediately and discuss the problem. You could also return the computer, assuming it's new enough. Maybe you want to RMA it?
I had a disturbing experience after purchasing an HP Pavilion notebook and tried to upgrade the memory. The particular model I purchased is configured with 512MB of memory. I was going to upgrade to 1 GB by buying a 512MB card with the assumption that there was a 512MB card already installed. After purchasing the card and opening the notebook to istall it, I find that the 512MB configuration was the result of using two 256MB cards instead of one 512MB card. There's only two slots in this model Pavilion notebook. Needless to say I have to buy two 512MB cards to upgrade (quite an expense and then have two useless 256MB cards). It seems to me that when they configure these notebooks with memory, there's no consideration of whether the consumer will upgrade and what would be the least expense to do that. They seemed to have used what was available at the time. A call to HP got me nowhere. Has anyone else had this same experience and did you get any relief from the manufacturer ?

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