After logging in as admin, I am unable to access the admin area.  The log shows the following:

2010-10-12 10:48:27,784 ERROR mojoPortal.Web.mojoBasePage - 70.57.95.86-en-US - /Admin/AdminMenu.aspx
System.Web.HttpException: Unable to validate data.
at System.Web.Configuration.MachineKeySection.EncryptOrDecryptData(Boolean fEncrypt, Byte[] buf, Byte[] modifier, Int32 start, Int32 length, IVType ivType, Boolean useValidationSymAlgo, Boolean signData)
at System.Web.Configuration.MachineKeySection.EncryptOrDecryptData(Boolean fEncrypt, Byte[] buf, Byte[] modifier, Int32 start, Int32 length, IVType ivType, Boolean useValidationSymAlgo)

I generated a machine key using the IIS Manager in IIS 7.  I have this error on two of my sites.  Two others don't seem to have the problem.  Not sure what the difference is.  Any ideas?  Thanks!

Yes, I tried clearing the cookies first.  Restarted my computer as well.  I don't get the error if I login from a Mac using Safari, but get the error coming from my windows vista computer running either Chrome or IE 8 in both the 32-bit and 64-bit versions.  I tried generating new machine keys as well and that didn't help.  I have the application in it's own app pool also.  I can't figure why I don't have the problem with a couple other installations on the same server. I've checked the web.config and can't see anything different except for different machine keys.  The mojoportal installation was done through Website Panel if that helps.  I tried logging in from an XP computer and was able to no problem so I'm thinking I must need to do some security updates on my Vista 64bit computer.  I thought it was doing automatic updates, but maybe not.  I'll let you know if installing the updates does the trick.  Thanks again for your help!  mojoPortal is awesome!

Neither clearing the browser cache or cookies helped.  I noticed that the website panel installed version 2.3.5.3, so I downloaded 2.3.5.4, deleted everything and installed that version manually.  It worked on one site, but on the other site with the problem, I get the following error now after logging in then clicking on one of the admin links: 

2010-10-13 11:29:55,850 ERROR mojoPortal.Web.Global - 70.57.95.86-en-US - /Default.aspx
System.FormatException: The input is not a valid Base-64 string as it contains a non-base 64 character, more than two padding characters, or a non-white space character among the padding characters.
at System.Convert.FromBase64CharArray(Char[] inArray, Int32 offset, Int32 length) 

Any other ideas?  Thanks!

I came back later in the day and it was working. Strange. I even ran ccleaner to clear files. I guess leaving the machine for a while cleared out whatever was still hanging around. Thanks for your continued help.