The best I have found is Shortcut Photozoom which used to be S-Spline 2. click here. There is trial version available so you can test it to see if it will do what want. You should bear in mind that enlarging an image beyond its original size will always result in loss of quality no matter what file type it is. It's more a matter of keeping the quality loss within acceptable levels.
You could use paintshop pro or any other photo manipulation program but make sure you resample the file to make it larger don't just resize it as this will give a blocky final image whereas resampling will smooth out the image to a certain degree.
As .jpeg is intended to reduce a substantial graphics file to a much smaller one, perhaps to send as an e-mail, by "losing" much of the detail whilst retaining overall appearance, it is clearly impossible to regain the lost detail once converted.
You therefore need the original file(s), if it is possible to obtain them, to do with as you wish.
It would certainly be possible to print them but even a small enlargement of the file(s) would be evident in the quality of the prints.
Sethhaniel, What Stuarli says is correct, but depending on the amount of compression used, it may be possible to enlarge by a small amount. Probably the best program would be Genuine Fractals. but don't expect miracles. Not sure if you can still download the free trial on this software.
Your best bet would be to enlarge it in any version of photoshop - you can pick a copy up very cheap at any local computer specialist. It wouldn't need to be an up to date version, a friend of mine is still using Photoshop 3 quite happily. Everything of course depends on how much info was thrown away when first compressed. Normally a web quality jpeg (under100k) is good for nothing much else.