Startup: desktop loads, then a few black screens appear

I have a new Dell Inspiron 1420 (about a week old) and the first few times I
started it up, everything was fine. Over the past few days, however, this has
been happening: the Dell logo loads, then the Microsoft Corp loading screen,
then the welcome screen, then the desktop loads and the cursor is stuck (I
can only move it horizontally). Sometimes it's fine and I can use the
computer, but inevitably a black screen or two will pop up before the desktop
finally loads 'properly'. Thus far this hasn't caused me to lose any work and
nothing has frozen up for more than a fraction of a second, but I'm still not
happy about the way the black screens interrupt my work.

I've checked for malware and viruses and all the reports have come up clean.
I've removed McAfee and installed only Comodo Firewall, MediaMonkey, Pidgin,
KMPlayer, Firefox, AVG Anti-Spyware and Avast Anti-Virus.

The problem only happens when I start up for the first time; it doesn't
happen when restarting (although I've only restarted once, so I can't be sure
if it was a fluke).