Hello.
I just compiled kernel 2.6.1 on Slackware 9.1, after reboot I get this message:
“VFS : Cannot open root device “307” or hda7
Please append a correct “root” boot option
Kernel panic: VFS : Unable to mount root on hda7. “
What can I do to fix this problem?

Firstly, we need your .config (should be /usr/src/linux/.config) and we need to know a little bit about your machine (specifically, IDE/SCSI, & partition / mount setup)

Usually this problem is something not set up correctly in your .config. When I was getting a similar error I forgot to compile support for IDE drives into my kernel. Make sure you have IDE / SCSI support (depending on what you use) and support for whatever filesystem your hda7 is (ext3 perhaps?)

This is part of config file:
......
# File systems
#
CONFIG_EXT2_FS=y
CONFIG_EXT2_FS_XATTR=y
# CONFIG_EXT2_FS_POSIX_ACL is not set
# CONFIG_EXT2_FS_SECURITY is not set
CONFIG_EXT3_FS=y
CONFIG_EXT3_FS_XATTR=y
# CONFIG_EXT3_FS_POSIX_ACL is not set
# CONFIG_EXT3_FS_SECURITY is not set
CONFIG_JBD=y
# CONFIG_JBD_DEBUG is not set
CONFIG_FS_MBCACHE=y
CONFIG_REISERFS_FS=y
# CONFIG_REISERFS_CHECK is not set
# CONFIG_REISERFS_PROC_INFO is not set
# CONFIG_JFS_FS is not set
# CONFIG_XFS_FS is not set
# CONFIG_MINIX_FS is not set
# CONFIG_ROMFS_FS is not set
# CONFIG_QUOTA is not set
# CONFIG_AUTOFS_FS is not set
CONFIG_AUTOFS4_FS=y

#
# CD-ROM/DVD Filesystems
#
CONFIG_ISO9660_FS=y
CONFIG_JOLIET=y
# CONFIG_ZISOFS is not set
CONFIG_UDF_FS=y

#
# DOS/FAT/NT Filesystems
#
CONFIG_FAT_FS=y
CONFIG_MSDOS_FS=y
CONFIG_VFAT_FS=y
CONFIG_NTFS_FS=y
# CONFIG_NTFS_DEBUG is not set
# CONFIG_NTFS_RW is not set

#
# Pseudo filesystems
#
CONFIG_PROC_FS=y
CONFIG_PROC_KCORE=y
# CONFIG_DEVFS_FS is not set
CONFIG_DEVPTS_FS=y
# CONFIG_DEVPTS_FS_XATTR is not set
CONFIG_TMPFS=y
# CONFIG_HUGETLBFS is not set
# CONFIG_HUGETLB_PAGE is not set
CONFIG_RAMFS=y

#
# Miscellaneous filesystems
#
# CONFIG_ADFS_FS is not set
# CONFIG_AFFS_FS is not set
# CONFIG_HFS_FS is not set
# CONFIG_BEFS_FS is not set
# CONFIG_BFS_FS is not set
# CONFIG_EFS_FS is not set
# CONFIG_CRAMFS is not set
# CONFIG_VXFS_FS is not set
# CONFIG_HPFS_FS is not set
# CONFIG_QNX4FS_FS is not set
# CONFIG_SYSV_FS is not set
# CONFIG_UFS_FS is not set

#
# Network File Systems
#
CONFIG_NFS_FS=y
# CONFIG_NFS_V3 is not set
# CONFIG_NFS_V4 is not set
# CONFIG_NFS_DIRECTIO is not set
# CONFIG_NFSD is not set
CONFIG_LOCKD=y
# CONFIG_EXPORTFS is not set
CONFIG_SUNRPC=y
# CONFIG_SUNRPC_GSS is not set
# CONFIG_SMB_FS is not set
# CONFIG_CIFS is not set
# CONFIG_NCP_FS is not set
# CONFIG_CODA_FS is not set
# CONFIG_INTERMEZZO_FS is not set
# CONFIG_AFS_FS is not set

#
# Partition Types
#
CONFIG_PARTITION_ADVANCED=y
# CONFIG_ACORN_PARTITION is not set
# CONFIG_OSF_PARTITION is not set
# CONFIG_AMIGA_PARTITION is not set
# CONFIG_ATARI_PARTITION is not set
# CONFIG_MAC_PARTITION is not set
# CONFIG_MSDOS_PARTITION is not set
CONFIG_LDM_PARTITION=y
# CONFIG_LDM_DEBUG is not set
# CONFIG_NEC98_PARTITION is not set
# CONFIG_SGI_PARTITION is not set
# CONFIG_ULTRIX_PARTITION is not set
# CONFIG_SUN_PARTITION is not set
# CONFIG_EFI_PARTITION is not set
.....

That looks good, what about the IDE section of your .config? (assuming you're running an IDE HDD partition mounted at /)

Also, if you are using initrd then you need cramfs support.