 RATIONALE: Drugs used in chemotherapy, such as temozolomide, work in different ways to stop the growth of tumor cells, either by killing the cells or by stopping them from dividing. Thalidomide may stop the growth of glioblastoma multiforme by blocking blood flow to the tumor. Isotretinoin may help cells that are involved in the bodys immune response to work better. Celecoxib may stop the growth of tumor cells by blocking some of the enzymes needed for cell growth. It is not yet known which temozolomide-containing regimen is more effective in treating glioblastoma multiforme. 