1. Dell Logo. Stop staring at the Dell logo during boot. Because you're having problems booting your system you should be viewing the Power-On Self Test (POST) diagnostic screen instead. You need to become familiar with what a normal boot looks like for your machine. If you get "lucky" you might see a difference for an abnormal boot (different messages, time delays?). Maybe it appears hung because it's taking time to count up and check the memory?
To view the POST diagnostic screen, try hitting the "Escape" key when the Dell logo appears. Hopefully this is the right key to take down the Dell logo and show you the diagnostic steps that happen during boot.
You should also change your BIOS settings via CMOS setup to display the most information available during boot. Within CMOS setup look for and set the following POST behavior parameters:
Fast boot : Off (Off - slows boot as more POST diagnostics are run)
Numlock key: On
POST Hotkeys: Setup & Boot Menu
Keyboard Errors: Report
The information above was taken from page 113 of your Operator's Manual:
http://support.dell.com/support/edocs/systems/dim5150/en/om/WD846A02.pdf2. Dell Diagnostics:
Your could try running the Dell diagnostics that originally came with your system:
http://support.dell.com/support/edocs/systems/dim5150/en/sm/tshoot1.htm3. Hard Disk Drive. Wild guess... I'd look toward the hard disk drive or its cabling. Maybe it's having trouble reading the disk... You could go to the disk drive manufacturer's web site, download and run the disk drive diagnostics most of them provide.
4. If you're logged in under your operating system, per page 33 of your owners manual, you may "RUN DISK CHECK":
1 Click the Start button and click My Computer.
2 Right-click Local Disk C:.
3 Click Properties.
4 Click the Tools tab.
5 Under Error-checking, Check Now.
6 Click Scan for and attempt recovery of bad sectors.
7 Click start.