Could be anything from a faulty tablet to no internal memory left because you've downloaded too much.
Go to >SETTINGS >SD CARD AND PHONE STORAGE
If the Internal Phone Storage is under 20MB you will be having trouble.
Go to >APPLICATIONS >MANAGE APPLICATIONS check > ALL and see how many you can move to the SD card (assuming you've pluggd a nice fat juicy SD card in)
Then go to > MANAGE APPLICATIONS and start deleting DATA and CACHE
Most tablets have a far too small internal memory and no way to move them to the SC card unless you root it. As someone who loves lots and lots of stuff this is irritating.
Having said that I'm very pleased with my cheapo Android tablet (the second one from Maplins, the first one didn't work and was replaced).