I'll try to make this as simple as possible.
In Inventor, go to your Help menu, then select "About Autodesk Inventor"
On the next screen you should see what build, Release and SP (Service
Packs, if any)
If you see "10 SP3a" then that means you are up to Service Pack 3a. If
you see something other than SP3a (none listed or SP1 or 2) then you
should install the Service Packs.
All of the SP's will have to be installed in the order they were
released. In other words, in order to install SP2, you will need SP1
installed first.
Depending on what "About Autodesk Inventor" displayed to you as
installed SP's, follow the links below as needed to obtain and install
any SP's you are missing.
link to Inventor 10 Series SP1 (Inventor10_ais_SP1.exe)
http://usa.autodesk.com/adsk/servlet/ps/dl/item?siteID=123112&id=5596411&linkID=9242019
There is a "Readme" file/link on that page that will tell you how to
install it.
link to Inventor 10 Series SP2 (Inventor10_ais_SP2.exe)
http://usa.autodesk.com/adsk/servlet/ps/dl/item?siteID=123112&id=5973751&linkID=9242019
Same thing with a "Readme" file/link here...
link to Inventor 10 Series SP3a (Inventor10_ais_SP3a.exe)
http://usa.autodesk.com/adsk/servlet/ps/dl/item?siteID=123112&id=6418565&linkID=9242019
Same thing with a "Readme" file/link...
After you've made sure you are up to SP3a, try to install/apply that
hotfix once more.
HTH