Windows 7 - Aftter installing new CPU, system won't boot

Hi, I recently bought an intel E5700 processor for my dell optiplex gx620 so I could upgrade it's performance. Currently, it has a Intel Pentium D 820 2800 MHz. (I'm a noob and I'm still learning little by little everyday about building, upgrading and configuring computers..so this is all a learning process.) So I open the cpu when it came in the mail and noticed that it comes with its only heat sink/fan setup...but the setup currently has its own heat sink attached to a fan thats attached to the rear of the case. I figured that I'll just switch the processor with the new E5700 and use the same heatsink and fan setup. I'm a noob so I don't know if I I'm supposed to take out the old heat sink and install the new fan/heat sink on top of the new processor as well as having the fan attached to the case running too. I didn't notice any inputs on the motherboard for the new fan, so thats why I figured I should use the old heatsink with the new processor..but after installing the new CPU with Zalman thermal paste, my CPU wouldn't boot up. So I put back in the old processor and here I am. I think I updated my bios..I went to the dell website to download the latest bios. b4 I checked that the latest bios were dated 2006..and after it rebooted and said bios updated, the date still says bios updated. Now I don't know if I should try putting the new CPU in or if I'm doing something else wrong. Any help please?

Here are the specs of my setup..

this is my motherboard: Dell OptiPlex GX620 PY423 Motherboard Foxconn LS-36

MSI R5450-MD1GD3H/LP Radeon HD 5450 1GB 64-bit DDR3 PCI Express 2.0 x16 HDCP Ready Low Profile Video Card

CPU TypeDualCore Intel Pentium D 820, 2800 MHz (14 x 200)

crucial 4gb kit 2-2gb 240 - pin unbuff dimm 256mx64ddr2 pc2-5300 ct3kit25664aa667

LITE-ON DVD Burner - Bulk 24X DVD+R 8X DVD+RW 12X DVD+R DL 24X DVD-R 6X DVD-RW 16X DVD-ROM 48X CD-R 32X CD-RW 48X CD-ROM Black SATA Model iHAS124-04 - OEM

Motherboard Chipset Intel Lakeport-G i945G

Disk Drive ST31000528AS ATA Device (1000 GB, 7200 RPM, SATA-II)

Computer Type ACPI x64-based PC

This is an Intel page for your 945G chipset:

Intel® 945G Express Chipset

The E5700 is NOT on the list.

Sorry that I can't be encouraging.

So the cpu I bought isn't compatible? Should I return or sell it?

I understand you are trying to upgrade your unit but first ,You must do LOTS and LOTS of reading my friend

Like previously said not to discourage you in anyway but I suggest reading some of the Forum Tutorials and take time to really understand what CPU is actually compatible and also how the heatsink of that particular chipset works ,other things i would include would be reading QVL's on any other upgradable items you have

These are things you really need to know before any upgrade can take place a potential wrong move can cause major damage

please keep us posted on any details you wish to discuss

Return the cpu but read what model you need for that chipset

ok thanks