Want to downgrade from Win 7 Home Premium 64 bit to 32 bit.

64 bit OS usually installs faster than 32 bit.  3GB of RAM is adequate for 32 or 64 bit and that should not be the slowdown problem.   Is the extra time the initialization and startup of the wi-fi link?  I notice it takes longer to boot up my Win 7 laptop and netbook because of wi-fi access.   Anti-virus programs can significantyly slow down startup of a PC.  Most new PC's come with McAfee anti-virus and that is a know bottleneck.  One thing that will speed up the startup is removing what antivirus you have now and installing the free Microsoft Security Essentials - free download from Microsoft.  I saw a big speed up on my desktop when I dumped McAfee and installed the Microsoft Security Essentials, which is rated better than McAfee or Norton in finding and removing viruses. 

If you want to install 32 bit, you will have to reformat the C drive then install Windows, Dell Laptop Support Software, chipset drivers and finally device drivers. 

HERE is the Dell instructions on a manual install, However, Dell DOES NOT have Windows 7 32 bit drivers which could be a major problem installing the 32 bit version.  Also, once you install a different Operating System, or Operating System version that what originally came with the PC, the Dell PC Restore partition on the hard drive is corrupted and you cannot use that to reinstall. 

Best option is to replace the Antivirus as I suggested and see if that improves the startup speed.  If that is you only issue, it's not really an issue.

Thanks for your help. My Dell laptop didnt have any anti virus program when I purchased it. I noticed that it took long time at start up and shut down at first too. After I installed McAfee, the start up and shut down speed is still the same. I tried to uninstall McAfee, and the speed is the same too. In general, it takes me long time to start and shut down the laptop so far regardless the anti virus program or wifi link. I just purchased it last week, I just have basic programs like Skype, printer, Microsoft Office, Photoshop. Thats all but it is still slow at start up and shut down. Any suggestion to solve the problem?

Just to add something to the discussion of 32-bit on this machine.  Even though Dell only shipped the 1555 with x64, all the drivers are compatible with x64 or x86, except for the Intel WiFi and Intel Integrated Graphics, but those are available on Intel.com.

If you just purchased the PC last week you may want to consider returning it and getting a different model.  Win 7 64 bit is much faster than 32 bit or older OS"s.  Faster boot up is one of the big things with Windows 7.  If you are in the US, you have 21 days from the Date of the Invoice (usually the date it was shipped) to contact Dell and request a return authorization.  After the 21 days it's yours to deal with.