Could not copy objDebugRealLifeScience.dll to binRealLifeScience.dll. Exceeded retry

After adding ajax control tool kit i am getting this error...

Could not copy "obj\Debug\RealLifeScience.dll" to "bin\RealLifeScience.dll". Exceeded retry count of 10. Failed.RealLifeScience

with  this one

Unable to copy file "obj\Debug\RealLifeScience.dll" to "bin\RealLifeScience.dll". The process cannot access the file 'bin\RealLifeScience.dll' because it is being used by another process.RealLifeScience

my project name is RealLifeScience

i clean the solution and then build again.then its was fine. bt sfter some time getting this error again....