I would definitely try to run R as administrator, both for installing the packages and loading them. This could solve your problem.
This probably has to do with file permissions. When you install the packages as admin in a location where only admin can read/write, running R as a normal user means you do not have the file permissions needed to load the package. Running R as admin will solve this, as admin does have the correct permissions.
Alternatively, you could install your R packages in a location where a normal user has read/write persmissions, e.g. C:/Users/UserName
(or something like that, I do not have my windows machine accesible right now).