Computer hangs at Welcome screen after entering password

Hello to the forum. I am having recurring problem with one tower in my network. Upon boot, the system gives me the login screen, I enter P/W, and the little blue circle greets me. So far, I have solved this by restarting with the reset key, telling the PC to start normally,and re entering my P/W.
I have also used system restore to reset the system to an earlier time, and seemingly, this works. I have also ran the Win 7 repair disc, but there has been nothing found on three separate attempts.
I have gotten around this issue by putting my PC asleep when I leave for any time, but I do not leave it on at night. The following is an event that showed up, and I do not know what it means.

Log Name: System
Source: Microsoft-Windows-HAL
Date: 2/1/2013 12:32:18 PM
Event ID: 12
Task Category: None
Level: Error
Keywords: (1)
User: N/A
Computer: Business
Description:
The platform firmware has corrupted memory across the previous system power transition. Please check for updated firmware for your system.
Event Xml:
<Event xmlns="Error">
<System>
<Provider Name="Microsoft-Windows-HAL" Guid="{63D1E632-95CC-4443-9312-AF927761D52A}" />
<EventID>12</EventID>
<Version>0</Version>
<Level>2</Level>
<Task>0</Task>
<Opcode>0</Opcode>
<Keywords>0x8000000000000001</Keywords>
<TimeCreated SystemTime="2013-02-01T20:32:18.451600000Z" />
<EventRecordID>138192</EventRecordID>
<Correlation />
<Execution ProcessID="4" ThreadID="2260" />
<Channel>System</Channel>
<Computer>Business</Computer>
<Security />
</System>
<EventData>
<Data Name="Count">1</Data>
<Data Name="FirstPage">80</Data>
<Data Name="LastPage">80</Data>
</EventData>
</Event>

Thank you for any help you might extend to this problem.

Thanks for the response and the great links. I have Malwarebytes Pro installed on the computer, along with ESET security suite 5. I have done the preliminary MEM test with the repair disk from Windows, but I will re-run the one from MEMtest. I couldn't find my copy. Thanks for giving the link.
Will try to remember to repost when I fix this vexing problem.

I'm back, more confused than before. Here is what I have performed, what I have changed, and what the result is:
Ran Memtest. No problems reported.
Ran ChkDsk. No problems reported.
Scanned with MalWarePro. Full scan in Safe mode. No problems.
Scanned with Eset Security 5. No problems reported.
Ran SFC. No problems reported.
Ran Repair disk for Win 7 32bit. No problems reported.
Restored from a previous restore point. Started fine first time, then went back into not booting past Welcome screen.
Restored from previous Windows Image file. Updated OS, Security, Etc. Same thing. Not wanting to go past the Welcome Screen..Does not have any problem in safe mode. Boots up, P/W entered, on to files.
Removed all hardware not essential to boot. (other HDs). Initially I was sure this cured the problem, but it was short lived. Two boots, and the problem came back.
As a last resort, I removed Password login and the PC booted right up. I am using it to post this.
I have over 50 PCs with windows 7 Professional loaded on them, and this is maddening. Waiting for the next shoe to drop.

I'd also run a few Startup Repairs to see if it finds anything.

If System Restore resolved this take a look at Updates being installed after that Restore Point which would be a prime suspect. Install them singly til you isolate the culprit, post it back here in our Updates forum or seek support via email on Windows Update website.

If not and you can boot work through the Troubleshooting Steps for Windows 7.

Otherwise the steps suggested for Troubleshooting Windows 7 Failure to Start

To all who assisted.
The culprit turned out to be a failing drive. I had the selection in the BIOS for checking hard drive health and it did. I just did not recognize it, and it was not the OS drive, but a second one. Never had this problem before.
Thanks to all who participated in helping out.