Problem with Screen and Locking Session on Inspiron 1501

I have an Inspiron 1501 model that was recently purchased. It came with Windows Vista Basic. After having trouble and problems with Vista, plus the fact that I did not like Vista all that much anyway (we'll try again after SP1); I downgraded down to homey Windows XP Pro SP2. That said, I used my drivers disks to install all the drivers for my hardware. Everything is great on that end. Then I noticed one little bug, more of an annoyance than anything more. In the Control Panel, I have it set so that when I close my lid, "Nothing Happens". Now, when I am logged on (I do not use Fast User Switching or the Welcome screen because I am the only user on my machine), and I shut my lid and open it again a little later, everything is perfect. Then came where I went downstairs for a while; and didn't want anybody touching my machine. So; I locked my session. Thus, I also shut my lid. I opened it later only to find that my resolution has changed from 1280x800 to 800x600. So, I logged on, where an ATI message briefly appeared for maybe 2 seconds, then disappeared. It didn't stick around long enough for me to read, but I made out that "You system's resolution has changed..." was included in the dialog message. So, I went into my Display Settings and changed it back to 1280x800. Oddly, how I did it was the following. The slider was still on 1280x800. But, obviously, the resolution was 800x600. Yet, the slider was still positioned on the correct setting. I had to move back one setting (to 1280x768), hit apply, then set it back to 1280x800 and hit OK to get my native resolution back. Now this problem ONLY happens when I lock my session, or log off completely. This does not happen when I shut my lid when I am still logged on or when my session is not locked. I personally have never seen this before, in all my years as a PC technician. Hopefully someone here can help me solve my case. Thanks in advance, Pete.