Cant Access Administrative Rights in Standard User Accounts

Hello, i m using Win7 Home Premium 64b, and till a few days back, i was enjoying a trouble free OS with a Standard User , along with an Admin one, where i always use the standard user account.
When installing and uninstalling softwares(in the standard user account), it will ask for Administrative Password, and once i enter it, it will be done. But since a couple of days back , if i try to install a new software , or use a software which requires Admin Authentication, instead of asking me Admin Password, it is giving me an error, saying "Administrative Rights Required, Log into the Administrative Account and try ". but this is irritating, as i require to log in into the admin account everytime, i need to use the application. What could possibly have gone wrong. How do i solve it?? Need help.
Thanks in advance
Jeebu

What did you do the day before this issue first came up? Did you join a domain? Change any settings?

Thanks Jamie and Pebbly for your quick reply. Dont exactly know what is the cause for this to happen, one possible cause can be that, i ve installed Tune Up Utilities, and it might have modified some files(correct me if i m wrong) while doing some maintainance.
I ve tried uninstalling the same to see whether, the changes it made would be reverted, but was in vain!

Regarding System Restore option, Actually my Restore Point is way long back, like six months old or so, and i ve installed so many Academic Softwares in the last few months. Since i m on vacation now, and need all of them now, Wont System Restore be a bad choice.??

Another Question: Is there any system file that i can change to get this administrative rights back?

Jeebu

Thanks a lot pebbly, i just did System File Checker, and Windows Update, and did a restart, and it looks like things are back to normal. Thanks once again. Cheers
Jeebu