There may be a few reasons for getting this error. I have done my best to outline (below) popular causes for getting an error when trying to install my templates.
1) CHECK YOU HAVE UNZIPPED THE FOLDER YOU DOWNLOADED:
I offer my templates as zipped (.zip) folders. The zip folder you download from suckmylolly.com contains a few files that have been compressed, to reduce their file size. Usually this folder will contain the blogger template - template.xml for example - my terms and conditions and an extra folder which contains the images. You cannot expect to upload this .zip folder and have it work - why?: because it's a compressed folder and not a blogger template. Uploading the .xml file will NOT (USUALLY) CAUSE ANY ERRORS but uploading the .zip file will not work. You must UNZIP this folder before installing the template.
If you don't know how to unzip files then I advise you to read this or this. I do not use Windows all the time but I believe that for Vista and XP - all you need do is to double click the file to unzip it (or right click).
2) TRY ANOTHER BROWSER:
I too get an unexplained error message sometimes when I test my templates. I have no idea why this happens but either waiting a little while or trying another web browser often fixes things.
3) ERRORS WITH WIDGET LABELLING:
If you can *almost* install the template (ie: it asks you to confirm that you're about to delete your widgets but then when you click "OK" you get an error) - then you might need to change something in the code. This error occurs because the widgets are labelled incorrectly and Blogger gets confused when you try and change the template. I have no idea why this happens to some people and not to others but it's really quite simple to fix:
1. Before making any change, you should backup your current template. This can be done by going to "LAYOUT" >> "EDIT HTML" and then clicking on "Download Full Template".
2. Make sure you have some sidebar elements - eg: Profile, Blog Archive, Followers - before trying to upload the new template (this is so that you get the 'Widgets are about to be deleted warning').
3. Go to LAYOUT >> EDIT HTML and attempt to upload the new .xml template again. This will then bring up a page which lists the widgets about to be deleted - this is normal when installing a new template. You are then asked to confirm this step. Instead of confirming, follow the steps below:
4. Press Control+F (find function) and search for this phrase within the big box of code:
You should find some code such as these:
5. Change the number "1" in the code to be a "2".
ie: so it would now look like this:
Repeat this step for all these that you find in the code (change all the 1s to be 2s)
6. Confirm and save. This worked for me. I realise that it's quite scary to change your blog html if you're not experienced with html code, but it's quite a simple change and this is also the reason I recommend backing up your template before making any changes. Unfortunately things do sometimes go wrong so you should be prepared.
Of course I only recommend trying this last method if you feel comfortable altering the html. You may have trouble uploading any customised template unless this is altered.
6. Confirm and save. This worked for me. I realise that it's quite scary to change your blog html if you're not experienced with html code, but it's quite a simple change and this is also the reason I recommend backing up your template before making any changes. Unfortunately things do sometimes go wrong so you should be prepared.
Of course I only recommend trying this last method if you feel comfortable altering the html. You may have trouble uploading any customised template unless this is altered.
