Do you remember what version?
The default location is c:\Ubuntu. Did you originally install it to the g: drive? If you moved the folder from its original location then it might work if you copied it back assuming the folder contents are still intact. Wubi is a windows application i.e wubi.exe. To reinstall just run the program.
However, Wubi is not recommended anymore since it has not been updated and there are some bugs and performance issues. You can install as a virtual machine if your PC has the capability or as a true dual boot configuration assuming you are willing to resize your existing windows partitions. Since it has been 2-3 years you might want to install the latest version of Ubuntu. If you are not willing to install a dual boot go with 12.04 LTS and Wubi.
https://wiki.ubuntu.com/WubiGuide